Abstract.1Competition is shown to be occurring within and between the congenersCoelopa frigidaandC.pilipes;it is noted that the two species frequently coexist despite ongoing competition.2Observations on natural wrack‐beds indicate that there is a marked difference in the distributions of the larvae:C.frigidalarvae aggregate in cooler parts of the bed,C.pilipesin warmer parts. This difference in microdistribution reflects a broader‐scale difference in the geographical distributions of the species ‐C.pilipesbeing the more southerly of the two.3The larval distributions are shown to be caused primarily by the behaviour of the larvae themselves ‐ not by choices made by ovipositing females, nor (at least to any great extent) by differential survival.4The different micro‐distributions within beds constitute a form of niche difference which will cause competitive abilities to be frequency‐dependent and hence have a stabilizing effect. It is possible that this effect may be supplemented by others; and, in particular, effects operating at the level of the‘linear meta‐population’may be worthy of further investigation both inCoelopaand in othe