Feeding, as estimated by egestion rate, in coastal epibenthic mud snails Hydrobia ulvae living continually submerged in lagoon-like conditions varies significantly with time of day. Both in the field and in the laboratory, egestion rate is maximal around mid-day and minimal at twilight. This variation, here ranging in magnitude from mean lows of some nine faecal pellets h−1 to highs of 34 h−1, is consonant with likely rhythms of availability and productivity of their microphytobenthic food.