The F/R-ratio (litres of water filtered per ml of oxygen respired) was determined for the filter-feeding demosponge Halichondria panicea to be 15.5 l H 2O (ml O 2) −1 which was used to evaluate the potential of the sponge to nourish solely on nano- (2–20 µm) and micro- (20–200 µm) phytoplankton cells in the sea. It was estimated that in order to balance the maintenance requirement of H. panicea the necessary content of suspended particulate organic carbon must be at least 0.03 mg C l −1, which may be compared with actually reported values of 0.04 to 0.2 mg C l −1 thus implying that H. panicea may be able to nourish on a sole diet of phytoplankton in nature. However, the amount of carbon represented by free-living heterotrophic bacteria, cyanobacteria and other small (0.2–2 µm) picoplankton which are also accessible to the sponge lies in the range of 0.05–0.10 mg C l −1, and therefore bacteria seem to be an important, although in many cases apparently a somewhat insufficient food source relative to phytoplankton. Video-microscope observations of the osculum cross-sectional area (OSA) and simultaneous measurement of the filtration rate of H. panicea showed that the filtration rate varied considerably over time concurrently with often pronounced variations in the OSA caused by disturbance when the aquarium through-flow was stopped during filtration rate measurements in the laboratory. It is concluded that the optimal and undisturbed filtration rate may be considerably higher than measured here, i.e. 6.1 ml water (ml sponge) −1 min −1, thus increasing the F/R-ratio to > 15.5 l H 2O (ml O 2) −1, which is comparable to values for more advanced eumetazoan filter-feeding marine invertebrates grazing on phytoplankton.