Qualitative and quantitative information on the smaller flagellates was obtained from one station in the inner Oslofjord in the period May-July, when the planktotrophic veliger larvae of the blue mussel (Mytilus edulis L.) have their maximum occurrence. This article presents the quantitative data for the pico- and smaller nanoplankton along with information on the occurrence of mussel larvae. The investigation was based on SDC (Serial Dilution Cultures) combined with light and electron microscopy assisted by epifluorescence microscopy. Species composition and flagellate concentration, as MPN (Most Probable Number), in the surface layers (0-10 m) as well as in the subsurface layers (10- 20 m) are reported. Information from SDC was supplemented by in vivo chlorophyll a measurements for different size fractions (<5, 5-10, >10 m). Flagellate taxa dominating the size fraction assumed to be the most important as larval food were Dicrateria inornata / Imantonia rotunda, Rhynchomonas nasuta, Nephroselmis pyriformis and several species of Pyramimonas.