Menidia clarkhubbsi, n. sp. is described as an all-female species on the basis of a unique combination of electrophoretic and standard morphological characters. At a phosphoglucomutase locus, M. clarkhubbsi is fixed for the presence of a unique electromorph which has not been found in surveys of other Menidia species. The new species also shows evidence of fixed heterozygosity at 6 of 29 electrophoretically examined genetic loci, and it shows extremely low levels of genetic variation. The electrophoretic data are explained by assuming that M. clarkhubbsi is a unisexual species that reproduces without genetic recombination. Preliminary evidence suggests a gynogenetic mode of reproduction. M. clarkhubbsi occurs commonly at two widely separated localities on the Gulf Coast of Texas where it is syntopic with M. beryllina and M. peninsulae. The all-female species typically has a long, opaque gasbladder, 8 dorsal fin rays and a horizontal distance between spinous-dorsal and anal fin origins of less than 7% of standard length. Those characters, in combination with sex, allow virtually complete separation of M. clarkhubbsi and M. peninsulae. The same combination of characters allows separation of about 92% of M. clarkhubbsi specimens from M. beryllina and about 99% of the latter species from the former.