This paper intends to recognize some aspects of interspecific relationships between Chrysomya megacephala (Fabricius) and C. albiceps (Wiedemann) and between C. megacephala and Cochliomyia macellaria (Fabricius) larvae in laboratory. A diet consisting of decomposing horse flesh was used, and the relation of 1 larva/g diet was established. The development of the two species was done in pure and associated cultures. The association with Chrysomya albiceps influenced the post-embrionary development period and the weight of mature larvae of the C. megacephala, reducing their. Such weight decrease, along with the predation of larvae of C. megacephala larvae, yelded a decrease in survival in all of the stages. Larvae of this species showed an increase in its weight when associated with Cochliomyia macellaria. The native species responded to the association by reducing its body weigth and its viability. This association doesnt influence the duration of post-embrionary development.