Ten population samples of the Crown-of-thorns starfish Acanthaster planci were collected between March 1982 and August 1983 from localities across the Pacific and were examined for variation at 14 enzyme loci using starch-gel electrophoresis. A sample of A. brevispinus was also examined for comparison. In contrast to the considerable genetic differentiation between A. brevispinus and A. planci populations (D=0.20+0.02), the genetic differences between geographic populations ofA. planci were small (D = 0.03 _+ 0.00; Fsr = 0.07 + 0.02), in spite of the great distances separating them. A positive correlation was observed between degree of genetic differentiation and geographic scale, suggesting that the genetic homogeneity among A.planci populations is due to gene flow by planktonic larval dispersion. In view of such macrogeographic homogeneity, it is striking that significant allele frequency differences were observed between adjacent populations separated by approximately only 10 km. The Hawaiian population was most differentiated from other populations. Treating the morphologically-distinctive, eastern Pacific Acanthaster as a separate species, A. eltisii, is not supported by these data. The lack of unique alleles in these two central and eastern Pacific populations suggests that they were derived from those in the western Pacific.