The annual infestation pattern of R. sanguineus sensu lato (Latreille) was studied for dogs in a tropical sub-humid region of Mexico. Infestation was monitored for a year at five veterinary clinics in different parts of the urban area of Colima City, Mexico. Dogs were considered positive for infestation when at least one specimen of any stage of the parasite was identified by physical search. R. sanguineus occurred throughout the year, with 21.4% average (446/2083; 95% C.I. 19-23) and infestation ranging from 17% in January to 29% in May. Seasonal distribution was homogeneous: spring 22.2% (96/432), summer 21.5% (103/477), autumn 21.4% (132/616), and winter 20.6% (115/558). No correlation was found between seasonal prevalence and weather during a season (p > 0.01). Tick specimens and all parasitic stages were found throughout the year. There were more larvae in March to June, September, and November; nymphs in March to May and December; and more adults in January, February, July, and October. Activity of each stage of ticks where dogs are always available indicated that R. sanguineus develop at least three generations per year.