The effect of order to hatching on the outcome of larval competition between two species of frogs breeding in 27 artificial ponds was studied. There were nine different treatments, each replicated three times: all combinations of various introductions of Bufo americanus hatchlings (none added, 500 added on day 0, or 500 added on day 6) and various introductions of Rana sphenocephala hatchlings (none added, 100 added on day 0, or 100 added on day 6). Response variables were the body size at metamorphosis, the length of the larval period of each individual, and the number of survivors of each species in each experimental pond. Bufo individuals and populations did best when alone. Also, they did better when introduced on day 6 rather than day 0. This may have been because the standing crop of food was greater in the communities that were 6 d older. When present with Rana, Bufo did better if added before Rana and worse if added after Rana, as compared to when both species were added at the same time. These results are consistent with a mechanism of size—specific competition. Rana also did best when alone and when introduced late rather than early. Rana did better when added after Bufo and worse when added before Bufo as compared to when both species were introduced at the same time. These results are not consistent with simple size—specific competition. When the species were together, both species did best when Bufo was added early and Rana was added late. These results suggest that optimal oviposition behavior is problematical for female frogs: the time that will be best depends on whether or not another species will be present at the time of hatching.