Abundance, cellular contents of carbon and nitrogen, and C/N ratio of planktonic thraustochytrids in the Seto Inland Sea and the coastal Hyuga-nada, Japan, were determined. Thraustochytrid cells were stained with acriflavine and counted directly by epifluorescence microscopy. Thraustochytrids were found in the water column at a density of 2.5 x 10(3) to 4.5 x 10(4) cells l(-1), with an overall average of 1.6 x 10(4) cells l(-1). In order to estimate the impact of thraustochytrid biomass on the coastal plankton community, cellular carbon and nitrogen contents of a cultured thraustochytrid were determined. The carbon and nitrogen for 10 mu m cell contents were estimated to be 1.65 x 10(-4) mu g C cell(-1) and 1.58 x 10(-5) mu g N cell(-1), respectively. The cellular C/N ratio of 10.4 was higher than that known for pelagic bacteria. The estimated population biomass of thraustochytrids varied from 0.41 to 7.44 mu g C l(-1), with an overall average of 2.60 mu g C l(-1); and 0.039 to 0.71 mu g N l(-1), with an overall average of 0.23 mu g N l(-1). The thraustochytrid carbon biomass corresponded to about 3.4 to 29% of total planktonic decomposers (bacteria and thraustochytrids). It is suggested that the contribution of thraustochytrids to the microbial food chain and carbon cycling through organic degradation and production may not be negligible.