Previous evidence has shown that benthic hydroids capture all kinds of available prey and the only known constraint was prey size. Among the prey captured are phytoplankton cells but it is not known whether they are digested and assimilated. To test the hypothesis that benthic hydroids assimilate phytoplankton cells, a series of feeding experiments was carried out with the Mediterranean species Eudendrium racemosum. Ingestion rates and assimilation efficiency were determined by analysing the 14C incorporated from a labelled population of the diatom species Thalassiosira weissflogii. Eudendrium racemosum fed on T. weissflogii, after a period of starvation, and with the diatoms as the sole food item. In the presence of approximately 15,000 diatoms ml−1, Eudendrium fed at rates ranging from 16 to 55 diatoms polyp−1 hour−1. Accumulation of radioactivity in the hydrocaulus and the polyps of the hydroids were observed. A maximum ingestion of 31.6 diatoms per μgC of polyp (i.e. 175 diatoms per polyp) was observed in the experiments. Most of the diatom 14C ingested would have ended up in the Eudendrium tissue (efficiency 94%), and it was expected that a certain percentage would have been respired by the polyps. These data show that Eudendrium feed on phytoplankton, which can satisfy almost 100% of their energy demand when this type of food is sufficiently abundant.