Field experiments were carried out to study larval feeding parameters ofSimulium cataractarumCraig 1987 (Diptera, Simuliidae) at the Trois Cascades de Faarumai (Tahiti, French Polynesia). Most larvae were attached to submerged moss on spring‐wetted rock walls beside the main waterfall. 1 I suspensions of red cellulose powder and two different mixtures of latex beads of various diameter were dispensed into the stream flow passing the larval colony. Gut‐passage time was calculated by the gut‐throughput of the red cellulose band. Particle ingestion was analyzed for artificial particles (latex beads and cellulose particles) as well as for natural food supply (diatoms and other algae). Particle capture efficiency was estimated calculating an absolute selectivity index IE* based on ingestion efficiency. By introducing this index estimation errors can be avoided if relative indices are used as demonstrated for Jacobs’ electivity index D which compares relative proportions of food categories or particle sizes in food supply and gut content.