A study of the effects in immatureCryptolestes turcicusof density and food quantity upon the rate of development, survival and the weight of newly‐formed adults has allowed the following conclusions:—(a) rate of development, survival and adult weight are highly correlated;(b) as food quantities per larva become smaller, so these values are reduced;(c) three intermediate density factors are recognised, one attributed to cannibalism, one to changes in the food medium brought about by the larvae themselves and a third which may be called disturbance. It is postulated that at least these three, acting independently or interdependently produce the response which is recognised as being due to density.Comparison withC. ferrugineusandC. ugandaeshows that the former species is more susceptible thanC. turcicusin response to high densities, whereas the latter is rathe