Food preferences (flavor and size) by two echinoderm plutei,Paracentrotus lividusandArbacia lixula, were studied in two experiments. These included selection between polystyrene spheres (flavored and non-flavored) of different sizes in short-term (15 min) feeding experiments and selection among food algae of varying sizes in long-term (24 h) feeding experiments. Clearance and ingestion rates change during larval ontogenesis and are dependent on the size and the quality of available particles. In general, clearance rates were higher for larvae fed on a mixture of three sizes of polystyrene spheres or algae than for those fed separate foods. Young larvae (4- and 6-arm stage) had a higher clearance rate with larger-sized spheres (18.5 μm) and larger-sized algaProrocentrum micans(26.0 μm). For older larvae (8-arm stage with rudiment), the preference was divided between medium- and larger-sized particles. Retention capability of small-sized spheres (2.9 μm) and algae(Isochrysis galbana), compared to the other sizes of particles, was very low during all experiments. When different sizes of spheres and algae were offered separately,Arbacia lixulalarvae ingested larger-sized particles at great rates throughout development. YoungParacentrotus lividuslarvae ingested larger-sized spheres at greater rates than other sizes of particles. During older larval stages, mediumsized spheres are ingested preferentially. In mixtures of different-sized particles, mediumsized spheres and algae are generally captured first. For young larvae clearance and ingestion rates increased when spheres were treated with a filtrate ofCriscosphaera elongataalgae. Young larvae select particles on the basis of their quality and size; as larvae grow, particle selection is size dependent.