The aim of this study was to identify possible infection of Toxoplasma gondii among cats in a shelter and a set of condominiums in the city of Rio de Janeiro, through changes to the cats' serological status between two different times in 2014 and 2015. One group was made up of captive cats at the municipal shelter and the other comprised stray cats that circulated in condominiums in the city. On the first occasion, cats were caught and tagged through application of microchips; in this manner, blood samples were obtained from 261 captive cats and 172 stray cats. On the second occasion, blood samples were obtained from 94 captive cats and 56 recaptured stray cats. The serological diagnosis was made by means of the indirect hemagglutination assay (IHA) and indirect immunofluorescence reaction (IFAT) (cutoff ≥ 64). The frequency of T. gondii infection among the captive cats was 24.5% and among the stray cats, 18%. With the second analysis, it was possible to verify modifications to the serological status of anti-T. gondii antibodies, in 18% of both populations of animals. The presence of seroconversion shows that infection was possibly occurring in the region at the time of the study.