WE have seen groups of small (4–5 mm long) grey flies belonging to the species Lipochaeta slossonae Coquillett, 1896, standing, shaking and apparently feeding, or flying for short distances on stretches of wet sandy beach in La Jolla, California and San Felipe, Mexico. When undisturbed, they walked sideways or stood and shook their bodies, diagonally forward and downward, and backwards and upwards, at an estimated frequency of 5 s−1. We guessed that they were fluidising the wet sand under their feet and thereby loosening some of the interstitial microflora which could then be sucked up as a kind of soup. This was confirmed by laboratory examination of the guts of several specimens, which contained the remains of large numbers of cells of dinoflagellates and diatoms. (The species of dinoflagellates were unfortunately unidentifiable since there were no cell wall remains, but they probably belonged to the genus Amphidinium, which comprises some of the commonest unarmoured interstitial dinoflagellates on this beach. The diatom species, however, could be readily identified by their silica walls. There were at least 10 genera, all typical of the marine interstitial community, including various species of Navicula, Nitzschia, Pinnularia and Amphora.)