ABSTRACT Genetic variation and population structure of the sea star Coscinasterias tenuispina were investigated using allozyme electrophoresis (seven polymorphic loci out of nine sampled) at seven sites on the coast of Rio de Janeiro State. Observed values of heterozygosity's were high (HOBS = 0.31 – 0.53), although similar to those found for others marine invertebrates. Departures from Hardy-Weinberg expectations were found only for one locus in three populations. No evidence of linkage disequilibrium was found among any loci for any population. The genetic identities vary from 0.882 to 0.988. The average θ was low and insignificant statistically (θ = 0.017). Furthermore, Bayesian analysis of population partitioning showed that the highest LnP(D) value was achieved for K = 1. These results do not corroborate any effect by upwelling limiting gene flow among the analyzed populations or the hypothesis that the strong sex ratio imbalance previously recorded to Itaipu population has been maintained by asexual reproduction.