Allelic isozymes of glucosephosphate isomerase at the Gpi-A and -B loci were separated by starch gel electrophoresis in the warmouth (Lepomis gulosus) and green sunfish (L. cyanellus). The specific tissue distributions and developmental expressions of the GPI-A2, -AB, and -B2 isozymes were not different between these two species. The synchrony of allelic expression in normal intraspecific sunfish crosses was demonstrated by means of an electrophoretic variant at the Gpi-B locus. In embryos formed from warmouth x green sunfish hybrid crosses, the paternal GPI-A2 isozymes were first expressed at the same time in both reciprocal hybrids, at 21-25 hr after fertilization. The maternal and paternal GPI-B subunits were synchronously expressed in reciprocal hybrids just for prior to hatching. The parental allelic isozymes at both loci shoed codominant expression in all tissues of the mature F1 hybrids. These results are consistent with the absence of allelic asynchrony and inhibition in interspecific hybrids formed from more evolutionarily related species.