Toxoplasma gondii infection of cats was studied in seven localities in Costa Rica. A total of 237 cats were tested for antibody by the dye test, and for oocysts by injection into mice. Toxoplasma was isolated from 55 (23%) and antibody was found in 109 (46%). Sixty percent of cats were infected as shown by shedding or antibody. Of the 55 isolations, 64% were from kittens without antibody and 36% from kittens with antibody. Almost half of the Toxoplasma isolations were made from the feces of kittens weighing less than 600 g, 35 cm in length, or under 2 months of age, of which 41% were shedding Toxoplasma. Six hundred and seventy-six homeowners in the seven towns were questioned about the number of owned and stray cats visiting their houses, the cats' pattern of roaming, food sources, types of food foraged, defecation sites outdoors and indoors, and location of fecal deposits in houses. Seventy-six percent of houses reported cats, which averaged 2.1 cats per house; about 80% were strays, and 99% lived outdoors part or all of the time. The cats visited from 5-12 (average 6) houses, foraging for food in garbage, hunting mice, rats, sparrows and insects. Of the 516 households reporting cats, 29% supplied some food in addition. In 14% of these houses cats defecated in the house, including the false attic and crawl space; they commonly defecated in front and backyards of houses, close to human habitation and accessible to intermediate hosts.