Ehrlichia canis is the etiological agent behind canine monocytic ehrlichiosis, and the tick Rhipicephalus sanguineus (Latreille) is its main vector. Blood smear and nested polymerase chain reaction (PCR) techniques were used to identify E. canis infection in dogs and R. sanguineus, and explore factors possibly associated with infection in dogs in Yucatan, Mexico. Blood samples were taken and ticks R. sanguineus collected from 50 dogs (10 house dogs and 40 in an animal control center). Data were collected on dog age, sex, body condition, and signs associated with platelet deficiencies (epistaxis). Blood smears were analyzed to identify E. canis morulae and generate platelet counts. Nested PCR analysis was done on blood samples and 200 ticks. A χ² test was done to identify factors associated with the E. canis infection in the tested dogs. The overall prevalence for infection, as determined by PCR, was 36% (18 out of 50). All positive dogs were from samples collected from the animal shelter, representing prevalence, for this sampling site, of 45% (18 out of 40). Morulae in monocytes were identified in only 4% of samples. Dog origin (i.e. animal control center) was the only variable associated with E. canis infection (P<0.01). Male ticks had a higher (P<0.05) infection rate than female ticks (24.5 vs 13.5%). It is concluded that E. canis infection is present in both dogs and the brown dog ticks R. sanguineus in Yucatan, Mexico.