Genetic variation at the mannosephosphate isomerase (MPI) locus was investigated in the Hawaiian spiny lobster Panulirus marginatus. Lobsters were collected over a 21/2 yr period (October, 1978 to March, 1981) from numerous sites throughout the Hawaiian Archipelago. Electrophoretic analysis revealed that this species exhibits a 3-allele polymorphism for MPI and that the proportions of the two most common alleles are very different in the two sexes. All adult males express a “slow” MPI allele, but females generally lack it. If one assumes that sex determination in this species involves an X-Y-like chromosomal mechanism, one can hypothesize that the Y (male-determining) chromosome carries only the “slow” allele while the X chromosome is polymorphic for all three alleles. Using MPI phenotype as a criterion of sex, I examined over 2 000 embryos and found a sex ratio of 1.08 males:1 female which was significantly different from 1:1. Thus, the sex ratio in the embryos is skewed toward males. Among puerulus post-larvae and adults there was also an excess of males. The slight but significant bias of the sex ratio toward males throughout the life cycle of P. marginatus is not readily explained by existing theories (e.g. mate competition or structured demes).