The invertebrates of fishpond were studied for 5 years after it had been cleared of fish. Five hundred Salmo trutta were then introduced. Species previously taken regularly but in small numbers and species taken occasionally were not recorded when predation by the fish was at its height. The common species, Corixa castanea and C. scotti were widespread when there were no fish and afterwards confined to places where vegetation provided dense cover. Nymphs were found in fish during the first year but rarely thereafter. A species with a short oviposition and quick growing period that overwinters as an adult cannot provide fish with much food. Species important from this point of view have a long breeding season or a long hatching season, or they establish a size hierarchy with a reserve of small species that do not grow unless they can take the place of a large one that has been removed.