We studied life history traits of Philornis seguyi García, 1952 (Diptera: Muscidae) parasitizing house wrens, Troglodytes aedon (Vieillot) (Passeriformes: Troglodytidae), and analyzed sexual differences in the size of larvae, pupae, and adults, as well as in the length of larval and pupal stages and in adult survival. Males were larger than females at the larval, pupal, and adult stages, but there were no sexual differences in the length of larval and pupal stages, or in the time adults survived. Larvae developed in 5–6 d and started the larval-pupal molt within 24 h after abandoning the host. Pupal stage lasted for 9 d and its length was positively associated with size of the pupa. The size of the pupa was positively associated with that of the larva and the size of the adult with that of the pupa. In laboratory conditions, 86% of the larvae pupated and 75% of the pupae emerged as adults. In natural conditions, the proportion of unsuccessful pupae increased by the end of host's breeding season. The proportion of males and females that survived until the adult stage did not differ from random. Our results show that P. seguyi has a relatively short generation time, which would allow it a rapid population growth during the host breeding season, and indicate that intrasexual selection may have selected for large body size in P. seguyi males.