A method for the direct measurement of picomole amounts of mono-, di-, and trisaccharides in seawater was developed by combining high-performance liquid chromatography (HPLC) with amperometric detection. This new method is described and then applied to the study of sugar uptake from seawater into veliger larvae of a bivalve (Crassostrea gigas) and a gastropod (Haliotis rufescens). Bivalve veligers take up glucose, maltose, cellobiose, and cellotriose but not L-rhamnose, isomaltose, maltotriose, or isomaltotriose. Similarly, gastropod veligers take up glucose and maltose, but not L-rhamnose or maltotriose. Glucose and maltose are transported into the bivalve veligers by separate pathways and the influx of "C-labeled glucose is a good measure of the net flux (measured by HPLC) The uptake of complex sugars, in addition to monosacchai-ides, permits inolluscan larvae to utilize a greater part of the dissolved organic material in the sea as a source of nutrition