We report the results of an 8-yr trap nesting study on the reproductive behavior of two cleptoparasite bees, Coelioxys funeraria Smith and Coelioxys moesta Cresson. This study provided a unique opportunity to examine parasite-host adaptation within a species, in which two different size classes of C. funeraria consistently laid eggs in the nests of the two different sized Megachile hosts, M. relativa Cresson and M. inermis Provancher. Additionally, we compared the behavior of C. funeraria to another Coelioxys, C. moesta, which also parasitized the nests of M. relativa. One striking pattern we found was the tight concordance of emergence times between hosts and parasites. The emergence patterns of individual C. funeraria parasitizing M. relativa nests closely matched that of the host, as well as that of C. moesta, which parasitizes the same host. These emergence patterns were significantly different from those of C. funeraria on M. inermis. We also found that Coelioxys and Megachile apportioned male and female offspring in the same parts of the linear nests and at similar times of the season. Female offspring tended to be placed in the innermost cells early in the season and males in outer cells later in the season. Because emergence patterns can severely affect offspring survival in these linear nesting situations, we suggest that the emergence times of males and females have determined the patterns of sex placement in both host and parasites.