A 17-month field survey and laboratory experiments were conducted to investigate the life cycle, seasonal population fluctuations, and salinity tolerance in the poecilostomatoid copepod Hemicyclops gomsoensis associated with the burrows of the mud shrimp Upogebia major and the ocypodid crab Macrophthalmus japonicus in the mud-flats of the Tama-River estuary, central Japan. On the basis of sample collections in the water column and from the burrows, it was revealed that H. gomsoensis is planktonic during the naupliar stages and settles on the bottom during the first copepodid stage to inhabit the burrows of U. major and, to a lesser extent, those of M. japonicus. While females carrying egg-sacs were present throughout the year, the copepods' reproduction took place mainly during early summer to autumn with a successive decrease from autumn to winter. Occasionally the copepod populations in the burrows suffered from severe flushes of river water that led to salinity decreases in the burrow water to fatal levels, but usually the salinity in the burrow was within optimal levels and permitted recovery and maintenance of the populations.