ABSTRACT: Penilia avirostris is an important component of the mesozooplankton of shelf waters during the stratification period. This study investigates the feeding ecology of P. avirostris under natural dietary conditions in the NW Mediterranean during their summer appearance. The results indicate that P. avirostris feeds on particles in a wide size range, mostly on nanoplankton (2 to 20 µm), including larger prey such as dinoflagellates and ciliates. Autotrophic and heterotrophic bacteria were not grazed on, but picoflagellates (<2 µm) were significant contributors to diet, indicating a narrow threshold of lower prey size. Ciliates were cleared at low rates which, at times, were not significant, probably because of their ability to escape feeding currents. Although this marine cladoceran is considered to behave like a filter feeder, our results indicate that it can display variable selectivity patterns that do not only depend on prey size. During the strict oligotrophic conditions that were present in summer NW Mediterranean waters, daily rations of P. avirostris averaged 82% body C d^–1 (range: 26 to 157%). Such feeding rates seem to be an adaptation to allow them to succeed under low food conditions and outcompete other components of the marine zooplankton, such as copepods.