The degree of infection with Paragonimus mexicanus of the crustacean Ptychophallus tristani was determined in an endemic area of paragonimiasis. Metacercariae were found in 161 (88.5%) out of 182 crabs. Comparing the infection in male or female crustacea, we did not observe any significant differences between both sexes. Size of the intermediate host was an important factor, since the infection was higher (94.4%) in bigger crabs; furthermore, the number of metacercariae was 2.1 ± 1.1 in smaller animals, while in big ones that number was 9.0±6.3 (P < 0.01). The infection of the crabs was season independent which indicates that infection sources are present through the year.