A small green sign inscribed with a white "100" stands beside the highway near a chain motel in Key Largo, Fla. Mile marker "0" is 100 miles south at the end of U.S. Rte. 1 in Key West. Between the two markers is an unusual stretch of road, an island-hopping series of bridges offering spectacular views of the sea.

Some people bicycle this 100-mile section, but most drive it. I decided to walk. If you have plenty of time, it's an unusual and interesting way to get to Key West.

The route is called the Overseas Highway, which links the 42 Keys that separate the waters of the Gulf of Mexico and the Caribbean Sea. The first day I covered 19 miles, but the rest of the way I averaged 13 miles a day, stopping at motels in the evening.

If there was any problem, it was the traffic. It snarls at you, particularly toward the end of the day. Trucks, uncountable vans, scores of Mercedes -- all trying to reach some warm-weather goal in the South -- tumbled down the highway at widely disparate speeds.

The original Overseas Highway, which was finished in 1938, recently has been rebuilt, but some of the old road still survives, paralleling the new road. Its pitted surface discourages cars, but it's fine for walkers and bikers and a welcome change from the traffic.

Some old bridges are still in place, too. That's fortunate because to get to Key West you must cross 40 spans, ranging in length from 37 feet to 35,716 feet (Seven-Mile Bridge). The new bridges have separate lanes for walkers and bikers, but the older bridges, which are closed to cars, are more pleasant. And there's always the opportunity to stop and chat with the fishermen who line them.

Along the way, I saw roseate spoonbills in a small mangroverimmed lake not 30 feet from the highway. And in the bike lane at the apex of Seven-Mile Bridge, while unseeing drivers zipped by at 60 mph, I paused for great views of the ocean to the east and Florida Bay to the west. To the north and south, the strand of islands that form the Keys glistened in the morning sun.

Then, there were the ospreys. Big birds with wingspans of five feet or more, they have white-striped heads, white breasts and gray-brown backs and wings. I saw more than 20 nests with ospreys in or near them. All were close to the highway, for these fish-eating hawks, in the Keys at least, prefer to build their huge, crude nests on telephone poles.

Ospreys ignore cars but emit a whistle that gets louder and higher if you approach on foot. Their friendly scolding kept me company during my walk -- as did the other birds. Kingfishers, with their belligerent-blue-jay silhouettes, sparrow hawks and cormorants jockeyed for space on electric lines. Egrets, ibis and great blue and little blue herons waded in shallow bays, lakes and canals.

I also saw Louisiana herons, dozens of cattle egrets and one great white heron. Pelicans, terns, turkey vultures and frigate birds filled the sky. I counted more than 25 species, and I am, at best, an amateur birder.

From the bridges, I enjoyed watching the fish. Dark, looming shapes swam in clear aquamarine waters: sharks, alligator gar, bonefish and rays.

You can't walk for long near this water without wanting to get in it. Fortunately, dive shops are as common along the highway as mile markers. For about $20, they will take you to a nearby reef for a half day of snorkeling or scuba diving.

In the Keys, two coral reefs, Key Largo and Looe Key, have been designated National Marine Sanctuaries. Friends recommended Looe Key, not a key at all but a reef seven miles south of Big Pine Key, where I joined Underwater Inc. for a half day of snorkeling.

The broad coral fingers of Looe Key extend from a central, shallow reef. I saw colorful reef fish, one wreck and -- in the sand spaces between coral fingers -- large numbers of barracuda. (Groups of 15 were common.) Curious, they would drift up to inspect me as I floated over; scared, I would rapidly flutter kick to the next coral finger though I have been told by countless divers that barracuda are harmless.

Afoot again, I reached Sugarloaf Key. On it, a few hundred yards north of Sugarloaf Lodge, stands an engaging monument to the 1920s. After World War I, a developer built a gambling casino, guest lodges and a large restaurant on the key, all of which are now gone. The only remnant is a cedar-shingled tower, one of two he built to house the bats he imported to eat the mosquitoes that plagued the island.

It's 17 miles from Sugarloaf to Key West. I made it in one day. I walked down Roosevelt Boulevard, which changes to Truman Avenue, turning right on Duval Street to Key West's Old Town. I dodged the tourists in front of restaurants, art galleries and trendy boutiques to find Mile Marker "0" at the waterfront on Mallory Square, the end of the road.

INFORMATION: Florida Upper Keys Chamber of Commerce, Key Largo, Fla. 33057; Florida Lower Keys Chamber of Commerce, Big Pine Key, Fla. 33050; the Greater Key West Chamber of Commerce, Key West, Fla. 33040.