Whether filter-feeding organisms can (1) select, (2) preferentially ingest, and/or (3) preferentially digest suspended particles, is of major importance to our understanding of material flow through marine systems. Investigation has until recently been limited by lack of techniques that can distinguish quantitatively between different particles of the same size. We demonstrate here the ability to distinguish algal food particles quantitatively, even when they are of a similar size, by the detection of their fluorescent photosynthetic pigments using a flow cytometer. The usefulness of this technique is illustrated by experiments on the mussel Mytilus edulis fed a mixed algal diet. In these experiments, a cryptomonad was digested in preference to a dinoflagellate and a diatom.