Our lack of knowledge of nanoplankton organisms is due largely, no doubt, to the necessity for examining them alive and the comparative difficulty of observing them, and also to their being, apparently, of no more than indirect importance in the ecology of fishes, except when their occasional presence in great abundance has been accompanied by unfavourable hydrographical changes. An adequate survey of life in the sea is bound to take them into account. Since they are frequently abundant, at least in inshore waters, and propagate rapidly, they probably form an important constituent of the food of adult filter-feeding invertebrates. They are likely to be of still greater importance as the essential food of many planktonic larvae, particularly in the early stages. Oyster larvae (Ostrea edulis L.) can ingest nothing larger than about 10 μ and appear to rely for food on minute flagellates (Cole, 1936,1939; Bruce, Knight & Parke, 1940). The food of other marine larvae has not been closely investigated.