Gut passage times in Gammarus pulex (Crustacea, Amphipoda) were calculated by counting and measuring voided faecal pellets . Following feeding on elm leaves conditioned in a stream, or summer fine detritus from the same source, calculated gut passage times were usually short, about 2 h or less at 15 ° C . Using harder oak and beech leaves, not previously conditioned in the stream, apparent gut passage times were much longer, 8-48 h, but since intermittent feeding occurred these figures are not comparable to the others .In connection with summer feeding of the animal in the stream, the microbiological status of the fine detritus there was studied . It contained fungal hyphae, which were mostly empty, viable fungal spores of terrestrial derivation, and bacteria, all largely carried on fragments of vascular plant tissues . In experiments, such fungal spores (of A ureobasidiumpullulans and of Mucor sp .), incorporated into fine detritus, were not digested by the animal . The bacterium Bacillus cereus was present in the fine detritus and when the latter was consumed it survived passage through the gut of the animal . Using B. cereus as a proportion marker it was concluded that other unicellular bacteria in the fine detritus were not digested either . In other experiments, not immediately related to the summer season, nutrient extraction by the animal from hyphae of the fungus Nectria lugdunensis was examined ; it probably occurs through sub-microscopic pores connecting the cells . This mode of extraction may apply when the animal consumes vascular plant tissues .