Relationships were determined between larval and pupal weight of Cochliomyia hominivorax reared in the laboratory on an artificial diet and in wounds in sheep. Six strains were studied (083, Micholima, A82, V81, Sinaloa and 009) in colony for 5-183 generations. Linear regression analyses of larval and pupal weight showed the 083 and A82 strains reared in the laboratory to be significantly different from each other and the other strains tested. This difference could not be attributed to length of time in colony or to geographic origin of the strains. There was no significant difference in the relationship between larval and pupal weights for the 6 strains reared in wounds. Composite regression lines for laboratory and wound-reared larvae and pupae were not significantly different. Mean weight loss from larva to pupa was 21 and 23% for wound and laboratory-reared screwworms. Survival from larva to adult was 91 and 94% for those reared in the laboratory and in wounds, respectively.