Streams on the island of Guam were surveyed to compare species composition and densities of the most common species among seven rivers, of which three were above a dam and reservoir and three were unregulated. Rivers above the reservoir were also compared to the outflowing river. Species found in all of the rivers included the indigenous gobiesAwous guamensisandStiphodon elegansand two introduced cichlid fishes,Oreochromis mossambicusandTilapia zillii.The gobies have modified pectoral fins that allow them to climb waterfalls and to traverse the spillway and enter the reservoir from downstream. The cichlids, however, are not morphologically adapted for climbing and were introduced to the reservoir; they eventually moved into the rivers. The freshwater shrimpMacrobrachium lar, smaller atyid shrimp of the generaAtyaandCaridina, and the gastropodThiara graniferawere also found above the reservoir, in the outflowing river, and in the unregulated rivers. However, the flagtailKuhlia rupestrisand the neritid gastropods were not found above the dam. In addition, the dam and reservoir appeared to affect the densities of some species. The densities of the mountain gobyStiphodon eleganswere significantly lower in streams above the reservoir than in unregulated streams. However, densities of the prawnMacrobrachium larwere somewhat higher in the streams above the reservoir, possibly a result of the exclusion of the flagtails. The effects of a small dam and reservoir with a spillway on the largely amphidromous fauna of tropical island streams are relatively benign, impinging primarily on species that are incapable of climbing the spillway or that are impeded in upstream migration by the reservoir.