Bill Adler is a little embarrassed to admit that he didn't immediately put two and two together when he heard the early-morning "pitter-patter of little feet" walking across the crawl space between the second and third floors of his house in Northwest Washington.

Adler, the author of several books about urban wildlife, said it took a few days of overhead noise before he realized three years ago that he had outdoor critters indoors. A family of raccoons had crawled up a tree, carved a small hole in the side of his house and moved into the Adler family home. A trapper was hired, the raccoons were evicted with the promise that they would be relocated, and the hole in the house was sealed up.

"They were so cute," Adler said. "They really were, and it's not their fault. You can't blame them for wanting to come inside."

This is the season when the outside world would like to get into your house, or under it. Foxes and skunks crawl under porches, squirrels and raccoons climb into attics, and snakes slither into basements and garages. Those habitats are warm and dry, compared with the increasingly chilly outdoors. Sometimes there is food -- that bag of birdseed will attract rodents, and a snake will catch their scent and want to come in, too. They don't need much of an opening. A quarter of an inch will do.

Late last month, a woman left her door open while working in the garden on a warm day. "The next morning, there was a raccoon sitting on her kitchen table," said Laura Oliver, field services manager for the D.C. animal shelter, which sent someone to shoo the animal out.

Animals are persistent. Raccoons, for example, can climb almost anything. They will make their way down a block of rowhouse rooftops looking for the easiest one to enter. Uncapped chimneys, unsealed vents and pet doors all offer an open invitation.

Fairfax County wildlife biologist Earl Hodnett recommends conducting a "critter access audit." Make sure your chimney is unoccupied and install a screened cap on it. Inspect the outside of your house, including louvers, electrical connections and pipe entrances. Have someone move a bright light around the attic, crawl space and basement at night, and have a second person stand outside to see whether light escapes. Adler's book, "Outwitting Critters," suggests that you tape tissue paper along the bottom stem of a clothes hanger "and hold the device near eaves, walls, windows you suspect are leaking air," which also could offer doorways for wildlife. (Fixing this has the happy side effect of adding weatherproofing.)

If an animal gets in, Hodnett says, learn its habits to get it out. It helps to know that squirrels leave during the day and raccoons do so at night, so that's the time to seal the entrance to prevent them from getting back in.

This assumes no babies are left behind while adults are out. John Griffin of A.C.E. Wildlife Services, which offers Humane Society-endorsed "ethical wildlife control," recently removed a nest of squirrels from a D.C. attic. With the mother out, Griffin put the babies in a "reuniter" -- a plastic container with a flap door containing nesting material tied to the roof near the animal's entrance point. He watched as the mother collected her babies in her mouth, one by one, and took them to a new den -- one of several she probably had scouted as backup.

If you decide to trap the animal, as Adler did, check traps frequently: The one at his house caught a neighbor's cat before it got his raccoon. The D.C. Humane Society will not leave a trap unless homeowners promise to check it every half-hour and close it before going to bed. The society also recommends that animals be released in the area where they are caught because it improves their chances of survival to be in a place they know.

-- D'Vera Cohn

The Humane Society of the United States has a winterizing checklist on the "humane living" section of its Web site, www.hsus.org.