Think of the ruins of ancient Italian cities, and most likely you'll think of Pompeii, near Naples. But there's a ruined city on the outskirts of Rome that rivals Pompeii: Ostia Antica.

Ostia, the first colony of Rome, was founded in the 4th century B.C. as a military outpost to control and defend the mouth of the Tiber. It soon became an important commercial port, one of the centers supplying food to the Roman army and the capital. In its heyday, Ostia had 50,000 residents, from officials, priests and families of the upper classes to merchants, artisans, workmen and foreigners from throughout the empire's provinces.

The city existed for about 900 years, until the 6th century A.D., but it began to decline after Constantine withdrew its municipal rights and gave them to another port city, Portus, in the 3rd century A.D. Building standards dropped and some parts of the town became slums. Raids, and the threat of attack by the Goths and other invaders from the north, hastened the deterioration, and the site was deserted completely in the 9th century.

During the 18th-century boom in archeology, British archeologist Gavin Hamilton started excavations in Ostia. The Vatican was interested in the patrimony of the region around Rome, and had the means of organizing the excavations. So it took over the project, beginning around 1801, when pope Pius VII ordered the first major excavations. They were continued later by the government, which took over new responsibilities as the Italian national state developed. Now, more than half the 160-acre city has been uncovered.

I visited Ostia on a Saturday morning not long ago, when it seemed all the Romans had escaped to their country houses in Tuscany or were spending the day at long lunches. I wanted to go to the country as well, but to someplace nearby and easy to get to. I had been in Ostia about 10 years before, remembered its fascination and decided to go again.

Ostia is among the better preserved sites of antiquity, in part because it was inhabited for many centuries and its marble wasn't stolen as was that at other sites. Parts were reconstructed in the 19th century; other parts are untouched.

Today, the city has a suburban feeling. Grass grows among the sandstone-colored ruins, and trees are set along the roads and next to the houses. Some roads are covered with paving stones, others are just dirt. Visitors enter through a gate about a 10-minute walk from the train station. You walk down the main street, Decumanus Maximus, which cuts through the center of the city.

Ostia was laid out along both sides of this street and divided into several sections: one a civic section that includes the Forum, shops and civic offices, the others residential and commercial areas. At one end of the street are ruins of the Jewish Quarter, separated from the rest of the city by a few hundred yards of high grass.

Ostia had a lively, heterogeneous population that is reflected in the variety of houses and public buildings they left. There are single-family homes with red and yellow wall paintings, atria and peristyles where the rich lived and apartment complexes where the middle class lived. There are public baths and toilets, a laundry, a tavern, the fire department barracks and rows of shops with back rooms and upper floors where the poor lived.

There are temples used by pagans, Christians and Jews, and the theater that amused them all. Commercial streets hold the warehouses, mills and business offices that brought in the city's wealth. It takes a full day to see it all.

The House of Cupid and Psyche, named after the small delicate statue of the two embracing, is a typical house of the rich, with a garden court edged in columns and marble floors in a design of circles colored in rose, cream, pale gold and gray.

But I found the apartments like the two complexes on the Via Diana more fascinating (the idea of apartment houses had always struck me as a modern invention for cramped cities). These three- or four-story buildings were often built over ground-floor shops and stalls, with stone staircases leading to upper floors, and courtyards placed in the center to allow light to enter from all sides.

Sometimes the houses had attached baths, but for Ostians without their own baths, the city offered public facilities. The Baths of Neptune are primarily in ruin, but you can still see basins, columns and the floor of the entrance hall with a mosaic showing Neptune driving four sea horses and surrounded by marine beasts.

The most curious remnants of Ostian sanitary life are the public latrines, two rooms lined with marble slabs in which about 20 holes had been cut about a foot apart. Underneath the seats are trenches that carried running water. The entrance on one of the latrines has a threshold where a revolving door had been, apparently to ensure some privacy.

The main business of the people of Ostia was trade, and the Square of the Corporations attests to the breadth of the commercial life. In the huge square are corridors lined with the rooms of 70 associations -- of the workers and businessmen who built and repaired the ships, maintained the warehouses and docks, loaded and unloaded ships, ran boat service on the Tiber, dived for sunken goods, milled and sold grain, dealt in rope, owned ships and handled commerce from all parts of the ancient world.

The front pavements are marked by black and white mosaics that show such designs as giant fish, ships entering the harbor or being unloaded, an elephant, a man riding a porpoise -- all insignia of the corporations.

The square is behind a huge, round theater, and some theorize that it was used as a promenade for the audience before plays and at intermissions, and that the rooms were not offices but club rooms kept by the associations as shelter from the rain.

But there's no doubt about the purpose of the ancient warehouse and mills some blocks away. A 64-room warehouse was used to store grains and legumes, and the flour mills across the street were kept busy grinding for the army and the markets of Rome and Central Italy. The millstones, which look a bit like figures out of "Star Wars," are two-piece contraptions made of lava. The upper part, a hollow double cone with holes through which the grain poured, was turned by donkeys or horses hitched to a bar.

Seaside Ostia, of course, had a good market in fish, and the old marble tank and selling table are intact. There is even an oven where fish that hadn't been sold could be cooked just before spoiling.

In the midst of "residential" Ostia is the fullonica (the laundry and dye place), still containing the basins that held water and the jars that held the dyestuffs.

My favorite place in town is the Thermopolium, a tavern on the Via Diana that has a lived-in feeling, so much so that you wouldn't be surprised to see someone walk out of a back room and serve you. At street-side is a marble counter for patrons who wanted to stop for a quick one, sitting on two stone benches just outside the door and watching the parade of Ostians pass by. Inside are three rooms -- one with a counter used to display dishes of food, with eggs, grapes, olives and radishes painted in reds and yellows on the wall above, a second for more formal dining and the third a kitchen with a round oven.

Ostian social life might have continued a few blocks away at the theater, which held about 4,000 patrons. Today it is rebuilt, with only the three theatrical masks from the marble decoration of the stage remaining of the original. (People here never seemed to get enough of the sea. Sometimes the orchestra was flooded to allow actors to stage mock sea battles and water shows.)

There was also plenty of religious variety. You can climb the broad stairs to the Capitolium, the most important temple, dedicated to Jupiter, Juno and Minerva. Or visit the long narrow stone chambers devoted to the cult of Mithras, the god of light and victor of good over evil. The Mithraeum of the Seven Spheres has an altar with a marble relief of Mithras plunging a dagger into the bull he rides.

The remains of the Christian Basilica preserve a baptismal font.

But the most extraordinary religious find was the synagogue. It was not discovered until 1961, and it takes some perseverance to find it even today. The Jewish ruins are at the southeastern end of the ancient city. You reach them by turning left before the Porta Marina at the end of the Decumanus Maximus.

The synagogue is a building of red and yellow brick sections. It was built in the 4th century A.D. over the ruins of another synagogue dating to the 1st century.Today you can still see the brick structure that contained the Torah scrolls. It is set between two columns with architraves that are sculpted with symbols of a menorah, a ram's horn, a palm branch and a lemon.

There are also four gray marble columns with Corinthian capitals and a vestibule paved in white mosaic. The other rooms of the building are an assembly hall, a bakery with an oven and marble counter for preparing unleavened bread, and the ritual bath, which got water from the small white well that still stands.

A marble slab with a 2nd century inscription says (the first line in Latin, the rest in Greek):

For the health of the emperor

Constructed and made at my own expense

The Ark placed for the Holy Law

Mindis Faustos

That plaque, like so much else in Ostia, reinforced the feeling I had that this was a place where real people lived, worked, paid obeisance to the man in power and hoped to earn a bit of credit. Seeing the plaque was a fitting end to my visit.