The rusty crayfish, Orconectes rusticus, has spread from its original range throughout much of northeastern North America, often completely replacing native crayfish. I tested the hypothesis that reproductive interference is a mechanism of the rusty crayfish's invasion of streams of the upper Susquehanna River watershed, where O. rusticus is replacing the native O. propinquus. I collected females from allopatric and sympatric sites during the spring reproductive period to measure size-specific probability of reproduction, fecundity, and the frequency of nonviable eggs. The predicted size at which females had a 50% probability of reproducing at sympatric sites was slightly larger for O. rusticus and considerably smaller for O. propinquus compared to allopatric sites, but O. propinquus also differed in size among site types. There were no differences between site types in average clutch size or the frequency of nonviable eggs. The differences in reproduction between allopatric and sympatric sites did not strongly support the reproductive interference hypothesis; instead, they may be a result of differences in O. propinquus size structure, possibly related to environmental differences between upstream allopatric and downstream sympatric sites.