Mud snails (Ilyanassa obsoleta) starved for 48 h were allowed to feed on sediments in laboratory microcosms. Sediment cores sliced at 2 mm intervals were compared to snail stomach contents for per cent carbon and nitrogen, plant pigment contents and species composition of benthic diatoms. Concentrations of carbon, nitrogen, phaeopigments, phycocyanin and chlorophyll were enriched in the top 2 mm of the sediments compared to 7-10 mm depth by a factor of 2-10. In turn, these materials were 20-40 times more concentrated in snail guts than in the surface sediments. Snail feces were enriched for carbon and nitrogen by 5-7 times over the surface sediments. Bacterial chlorophyll peaked at about 3-4 mm in the sediments and was not detectable in the snail stomach contents. The C/N ratio of the snail stomach contents was only 6 compared to a ratio of 8.5 for their feces and 12 for the surface sediments.The percentage of migratory diatoms (e.g. Nitzschia and Navicula) decreased with depth where non-migratory species, such as Fragilaria pinnata, dominated. These migratory species were more common in the snails than in the sediments on which they were feeding.A comparison of daily ingestion rates to the animal's energy budget shows that this selective ingestion is sufficient to meet Ilyanassa's energy needs.