C. gigas X C.rivularis. Staining patterns for several enzymes (EST, MDH. AndMPI) differed sufficiently between the two species to create a distinctive hybrid pattern in the experimental progeny. On the basis of one or more of these enzymes, we were able to identify 28 of 29 F1 as hybrids in experiment I, 29 of 29 as hybrids in experiment II, and 25 of 25 progeny as hybrids in experiment III. A single individual in experiment I showed C. gigas patterns at these enzymes, suggesting it was a contaminant.