The pre‐adult developmental periods of both sexes and oviposition rate of females ofCryptolestes turcicus(Grouvelle) have been studied at different constant temperatures and relative humidities upon wheatfeed. It is concluded that populations can maintain themselves only within the temperature range 17°C–37°C. and at relative humidities well above 40 per cent. The most rapid rate of increase is probably achieved near 28°C. 90 per cent R.H. Differences in biology betweenC. turcicus, C. ferrugineusandC. capensisare discussed in relation to their natural occu