Laboratory microcosm experiments reveal that benthic microalgae (filamentous blue-green and diatoms) probably constitute the bulk of nutrition in the somatic growth of the deposit feeding mud snail Hydrobia totteni. Despite an apparent excess of nitrogen in the particulate fraction of the sediment, H. totteni grows only about 42% as much in the dark as in the light. Growth in the dark is probably explained by the utilization of filamentous blue greens and, to a lesser extent, bacteria. The additional growth in the light is probably explained by benthic diatoms, as shown in previous studies. Standing-stocks of micro-organisms (bacteria and microalgae) and snail densities (0.5 cm–2, 2.0 cm–2) in laboratory treatments lie within the range of field conditions. A comparison of snail growth in oxidized and nonoxidized sediments, shows that particulate organic matter typically found in salt marsh sediments does not contribute substantially to somatic growth in H. totteni. Added rations of the seaweed Ulva rotundata also did not affect snail growth. Much of the available nitrogen may have been mineralized into the water column because Ulva is readily decomposed.