Feline ehrlichiosis is caused by gram-negative, intracellular and pleomorphic bacteria that reside on platelets or leukocytes. Although Ehrlichia canis in dogs is widely studied, there are few reports of its occurrence in cats, especially in Brazil. The present study aims to diagnose domestic cats in Rio de Janeiro naturally infected with Ehrlichia sp. by serological and molecular methods, and evaluate hematological disorders associated with the infection. Two hundred and nineteen domestic cats of the metropolitan region of Rio de Janeiro state were sampled. The detection of IgG anti-E. canis antibodies was performed by immunofluorescence assay (IFA), using as cutoff 1:64. Molecular diagnosis of Ehrlichia sp. was performed by nested polymerase chain reaction (nested PCR), based on a 350bp fragment of the 16S rRNA gene. Sixty animals were seropositive by IFA. In PCR, 37 animals were positive for Ehrlichia sp., of which 18 presented inclusions in platelets and/or leukocytes by direct detection in blood smears. On the other hand, 12 animals were shown to be concomitantly positive for IFA and nested PCR. Two samples positive in nested PCR were sequenced, in which one demonstrated 100% similarity with E. canis and the other 99% with Ehrlichia sp. ocelot. Hematological alterations most frequently observed among animals positive for Ehrlichia sp. were normochromic normocytic anemia, leukocytosis, neutrophilia with left shift and thrombocytopenia. These findings confirm the occurrence of Ehrlichia sp. among cats in Rio de Janeiro, Brazil. Therefore, future investigations become necessary in order to better characterize this agent and the issues involved in its transmission.