Population structure of the California grunion, Leuresthes tenuis, was asssessed by examining patterns of allozymic variation. Six specimens from each of four southern California locations (Goleta, Malibu, San Pedro, and Imperial Beach) were examined electrophoretically for the gene products of 55 presumptive loci; nine loci were found to be polyallelic. Direct-count heterozygosity averaged 0.036. A total of 50 specimens from each of the four locations was scored for the gene products of the nine polyallelic loci. A conservative significance test using exact probabilities showed that all four samples individually agreed with Hardy-Weinberg expectations. Few private alleles were detected, and Nei's genetic distances between pairs of samples were negligible. F-statistics were not significantly different from zero. However, a contingency chi-square test including all four geographic samples falsified the null hypothesis of no significant difference among the samples (P = 0.038). A Mantel test showed a significant correlation between geographic distance and Cavalli-Sforza and Edwards' chord distance matrices. The population structure approximates panmixia but also contains some indication of isolation by distance.