In sexual species, speciation occurs through the accumulation of genetic barriers to gene flow. In Caenorhabditis briggsae, one such barrier impedes gene flow between temperate strains and the tropical AF16 strain. Up to 20% of F2 progeny derived from crosses of AF16 to strains from the temperate clade exhibit a delayed development phenotype. This phenotype, which results from dysgenic interactions between maternal- and zygyotic-effect loci, causes a ~21% decrease in the intrinsic growth rate. The maternal-effect requires contributions from both parental genotypes. The dysgenic maternal-effect allele appears to be fixed in the temperate clade of C. briggsae and appears to have arisen between 700 and 15,000 y ago. The dysgenic zygotic allele appears to be present only in AF16 and also may be of recent origin.