Chromosome numbers of n = 19, 2n = 38 are reported for Mercenaria mercenarla, M. campechiensis and their F1 hybrids. Meiosis is normal in the hybrids and yields no evidence of chromosome nonhomology or structural rearrangements between the 2 parents. The hybrids produce functional eggs and sperm which result in normal fertilization and early embryonic divisions in reciprocal backcrosses and at least some F2''s. No gross chromosomal barrier to gene exchange appears to exist between the 2 species.