In this paper I combine experiment and theory to demonstrate that aggregation of carrion flies ovipositing among carcasses may facilitate coexistence in nature. The paper is divided into an experimental part and a theoretical part. In the experiment part, I present field experiments that show the aggregative pattern of carrion flies ovipositing among carcasses, as well as associated experiments that explore the behavior leading to these patterns. I also use experiments to quantify the strength of larval competition between Phaenicia coeruleiviridis (Calliphoridae) and Sarcophaga bullata (Sarcophagidae). In the theoretical part, I develop two methods that use these data to assess the effect of aggregation on competition in the field. From the first method, I predict that intraspecific aggregation of P. coeruleiviridis females causes a 26% decrease in the expected recruitment of P. coeruleiviridis and a 74% increase in the expected recruitment of S. bullata. As a result, within a single generation, intraspecific aggregation increases intraspecific competition and reduces interspecific competition. The second method for assessing aggregation involves a new mathematical formula that gives conditions under which species may coexist indefinitely. This formula requires estimates of only the degrees of intra— and interspecific aggregation among species; there is no need to estimate adult survival and larval competition. I use this second method to show that the minimum effect of aggregation on coexistence for five carrion fly species is equivalent to reducing the amount of larval competition between pairs of species by an average of 57%. Aggregation may therefore play a major role in explaining the coexistence of carrion fly competitors.