The natural diet and in situ feeding frequency of the hydroid Eudendrium racemosum, common in certain western Mediterranean shallow-water benthic communities, have been studied by analyzing their coelenteron contents. One hundred polyps were sampled, and their contents studied, every 2 h over three 24-h cycles. The overlying Zooplankton was simultaneously sampled in order to compare the prey with the available plankton. The maximum residence time of the prey in the coelenteron was estimated to be ≈ 5 h. Eudendrium is carnivorous, each polyp taking an estimated mean of 4.3 prey items · day-1. Thirty-nine percent of the polyps had at least one prey item at all times. Prey items had smaller dimensions than the polyp, but long thin organisms were sometimes taken. The number of captured items was strongly correlated with Zooplankton density, suggesting that E. racemosum is an opportunist with no innate diurnal or other feeding rhythm. Estimated captures of E. racemosum are of 100000 prey items · m-2· day-1 in the study area. Therefore, benthic hydroids may exert considerable effects on local plankton populations and play an important rôle in the transfer of energy from the plankton to the benthos. © 1988

13 pages, 3 figures, 6 tables