During aerobic decomposition of a natural assemblage of particulate organic matter collected by a 70μm-mesh nylon net from the north basin of Lake Biwa, a rapid regeneration of both ammonia and phosphate occurred in association with the proliferation of bacteria and heterotrophic microflagellates. Although ammonia was liberated during the growth periods of both bacteria and microflagellates, phosphate was mainly regenerated in parallel with the proliferation of microflagellates. This suggests the enhancing effect of microflagellate activity on the regeneration of phosphorus from indigenous particulate organic matter.