Cats are important in the epidemiology of Toxoplasma gondii infection because they are the only hosts that can excrete the environmentally resistant oocysts. Antibodies to T. gondii were determined in serum samples from 220 domestic cats (Felis catus) from Barcelona, Spain, using the modified agglutination test (MAT). Antibodies to T. gondii were found in 99 (45%) of 220 cats, with MAT titers of 1:25 in 26, 1:50 in 57, and > or = 1:500 in 16 cats. Seropositivity (MAT 1:25 or more) was significantly higher in adult (> or = 1 yr old, 49.7% of 153) than in juvenile (< 1 yr old, 34.3% of 67) cats, in feral (51.9% of 131) than in domiciled (34.8% of 89) cats, and in cats living in a group (community) of more than 5 cats (50.7% of 142) than in cats living alone (28.0% of 50). These seropositive cats are likely to have already shed T. gondii oocysts in the environment around Barcelona.