The rate of pupation of Tribolium freemani larvae was greatly influenced by the population density but not by the conditioned diet and its diethyl ether or water extracts. Larvae at high densities (over 20 per 2g wheat feed per vial) failed to pupate for more than 6 months, while those at lower densities (below 5 per 2g wheat feed per vial) pupated within one month. Isolation of the larvae that had been persisting in high density vials resulted in pupation within a short period of time. The prevention of pupation by increased larval density would be a main regulatory factor for controling the population size, although cannibalism as in T. castaneum cannot be disregarded. The primary factor causing this density effect is discussed with respect to the endocrine secretion resulting from physical contacts with other larvae.