We have attempted interspecific hybridizations among six species of rhabditid nematodes: Caenorhabditis elegans, Caenorhabditis briggsae, Caenorhabditis remanei, Caenorhabditis sp. v, Rhabditis sp., and Pelodera teres. Copulation was observed in all crosses between Caenorhabditis species; however, none resulted in the generation of stable hybrid populations. No copulation was observed in crosses between Caenorhabditis males and Rhabditis or Pelodera females, even when congeneric females were present, suggesting that Caenorhabditis males are able to selectively recognize congeneric females by a short-range stimulus. All pairwise combinations of Caenorhabditis species were isolated to some degree by gametic mechanisms; 7 of 12 combinations were cross infertile and 5 of 12 were cross-fertile but had low brood sizes. In cross-fertile combinations, most hybrid embryos were inviable and arrested prior to gastrulation. Only in crosses of C. briggsae males to C. sp. v females did any hybrids survive embryogenesis. Most of these C. briggsae/C. sp. v hybrids arrested during larval development, and the few that reached adulthood invariably were female. These results are consistent with the presence of at least two lethal factors in the C. briggsae-C. sp. v combination: a maternal lethal factor in the cytoplasm of C. briggsae and a recessive lethal factor on the X chromosome of C. sp. v.