Cats from several sources in Baltimore, Md, were tested for seropositivity to Rochalimaea henselae and R quintana. Co-infection with Toxoplasma gondii or feline immunodeficiency virus was assessed as a risk factor for infection with Rochalimaea spp. Of 592 cats tested, 87 (14.7%) were seropositive for one or both Rochalimaea spp, although titers to R henselae were significantly higher than those to R quintana. Prevalence of seropositivity increased significantly with cat age and weight and was associated with seropositivity to T gondii but was not associated with gender. Prevalence of seropositivity was similar (12.5 to 14.4%) among groups of cats with some history of human contact but was higher among feral cats (44.4%). Whether cats are reservoirs or mechanical vectors of Rochalimaea spp that can cause diseases in people is still uncertain, but these findings indicated widespread infection of cats and suggested possible modes of transmission for Rochalimaea spp among cats.