Observation on the shore show that the density of the population of the prosobranchHydrobia ulvaeand of the bivalveMacoma balthicaat Whitstable varies considerably With the grade of deposit, dense populations being found in fine deposits and sparse populations in coarse deposits. From laboratory experiments it seems probable that these animals feed by abstracting proteins, which can be assessed as nitrogen, from the bodies of microorganisms in the deposits and that the micro‐organisms are more abundant in fine deposits than in coarse ones.As a result of these conclusions it is suggested that the rapid increase in population density ofHydrobia ulvaeandMacoma balthica, and by inference of other deposit‐feeders, towards regions of fine‐grained deposits is attributable to the increase in density of the micro‐organism population. this, in turn, is related to the surface area of the deposits rather than to the abundance of organic debris.Finally, the nature of detritus is discussed and it is recommended that this term is replaced by “organi