Three species of mangrove crabs (Scylla spp.) were captured in live traps in Ngaremeduu Bay on the island of Babeldaob, Republic of Palau. Most were S. serrata, but one individual each of S. olivacea and S. paramamosain was also trapped, establishing existence of a biogeographic gradient in mangrove crab species diversity across the Micronesian archipelago. Species composition of mangrove trees along transects around the bay and along the three major tributaries was similar to that of other Micronesian islands, although trees are smaller in Palau. For 17 months in 1999–2000, crabs were trapped in the bay and captured by hand along the transects; they were trapped again for 1 month in 2004. Characteristics of the crabs and of burrows encountered along the transects suggested that only S. serrata was captured in 1999–2000 and that population density of this species was 40 crabs ha^−1. Carapace widths for the 159 crabs captured during the entire study did not differ significantly over the 4-yr span, and averaged 153 mm for males and 137 mm for females. However, average carapace widths for the largest quartile of crabs declined significantly from 174 mm to 171 mm across the study period. Catch per unit effort was 0.28 crab per trap night in 1999–2000 and 0.45 in 2004. Although large crabs are still available in Ngaremeduu Bay, current regulations may not be sufficient to keep populations from decreasing gradually in size, especially in the face of increasing harvest pressure on the island of Babeldaob.