Five species of non-pigmented microflagellates (3-10 μ m). isolated from the York River estuary, Virginia, U.S.A., were successfully cultured. All five microflagellates were shown, by feeding experiments and electron microscopy, to ingest live bacteria. These same microflagellates were not capable of utilizing 11 organic substrates at concentrations to 0.75 mg · 1-1. We propose that the normal nutritional mode of the marine microflagellates tested is to ingest bacteria rather than dissolved organic matter or a combination thereof. © 1979.