ABSTRACT: The abundance of thraustochytrids was investigated in and off the Seto Inland Sea, Japan. Thraustochytrid cells were stained with acriflavine and counted directly by epifluorescence microscopy. Thraustochytrids were present in the water column at adensity of 2.1 x 10³ to 5.6 x 10⁴ cells l^-1, with an overall average of 1.0 x 10⁴ cells l^-1, which was about 10^-5 to 10^-6 of the bacterioplankton abundance. In contrast to thelow abundance of thraustochytrids, their biovolume was estimated to compose a not negligible fraction of marine microbiota, corresponding to about 3 to 43% of the bacterioplankton biovolume. Thus, it is suggested that thraustochytrids play an ecologicalrole as alternative food sources for picoplankton feeders, as well as being active degraders and consumers in aquatic microbial food chains.