Flour beetles, Tribolium castaneum (Herbst) and T. confusum (Duval), have been the subject of many studies in ecology and genetics. The most important demographic attributes of Tribolium are fecundity, fertility, development time, survivorship and dispersive proclivities, and flour beetles also exhibit cannibalistic behaviour (intra-species and inter-species predation) which is important in population regulation (Mertz 1969; Young 1970). The magnitudes of these attributes vary with the genetic strain and age and sex of the beetles and are also affected by the collective properties of their environment. The presence of a second species affects cannibalism (Park, Mertz & Nathanson 1968; Park, Nathanson & Ziegler 1970) and the distribution of individuals within the environment (Nathanson 1971). The fecundity and cannibalism rates are dependent on population density (Rich 1956; Sonleitner 1961) and on the nutritional state of the medium (Sonleitner 1961). In these and similar studies the experimental conditions were varied simultaneously with the measurement of the demographic variables, and the effect of prior environmental conditions generally were ignored. Experiments with animals other than Tribolium show that different populational or environmental conditions imposed at one stage may exert a persistent influence through later stages. Barker & Podger (1970) showed that Drosophila, when crowded as larvae, had a lower fecundity than those not crowded. Murdoch (1966) has examined seasonal reproduction in the carabid, Agonum fuliginosum (Panzer), and suggests that episodes of high reproduction in favourable times entail a compensatory decrease later on. Frank (1960) demonstrated that for Daphnia, the recuitment in previously crowded populations was lower than uncrowded after both were diluted to the same density. This persistent effect-reflecting a previous experience of the population-he termed a 'lag' phenomenon due to 'prior history'. Tribolium beetles sampled from populations of different densities and subsequently cultured under the same conditions might also exhibit differences because of the prior histories. Further, if past events alter later physiological or behavioural states of any animal, there arises the theoretical question of the extent to which measurement over a short interval in present time can compensate for ignorance of past history of the population. These experiments measured the effects of prior histories on Tribolium and also provide an example of the importance of such effects in models of population dynamics.