Since it is common in forensic entomology for investigators to consult data on the development of flies when estimating postmortem interval, the results of the current study suggest that such data will not produce helpful estimates if the larvae growth on which they are based fed on a type of animal tissue other than that on which larvae fed on the corpse being examined. Forensic practitioners should note the part of a corpse from which larvae are collected and take into account any differences between that food substrate and the food substrata of the larvae used as the reference for time-related development stages. The larvae of both fly species lagged in growth when they fed on sheep's liver compared with the larvae that fed on meat and brain. Apparently the main factors that contributed to the observed differences in growth rate were the food's tissue structure, the activity of proteolyic enzymes, and the amount of soluble protein; however, other nutrients might also have played a role in larvae development. In addition to being smaller than the other larvae, the larvae that fed on sheep's liver molted later, reached maximum length more slowly, and sometimes produced significantly smaller pupae. Larvae in the study were grown on sheep's liver (representative of the commonest fodder type in published developmental studies), lamb chops (possibly similar in structure to a wound environment), and sheep's brains (to examine growth of larvae that infest the brain cavity). 2 tables and 35 references