The study of population ecology is, in the final analysis, concerned with the interaction of three basic factors, namely; natality, mortality and dispersion. This is true of work in the field or in the laboratory; whether the approach is (concerned wth census trends), analytical (concerned with causal relations), or deductive (concerned with mathematical models). Natality, mortality and dispersion produce shifts of numbers in time which are observed in the reproducing population. Flour beetles of the genus Tribolium are particularly favorable material for laboratory population study. Technical advantages and ecological characteristics make them suitable for detailed investigations of numerous problems at various levels of complexity. Some of the work which has been done deals with the relationship of a particular response and a particular factor or set of factors; for example, density and fecundity, or temperature and developmental rate. Such studies would generally be classed as analytical. More populational are the studies in which the response of an entire population or mixed-species population is observed. The work reported here is part of a program which is essentially an analytical study of the mechanisms operating within a population and resulting in what may be regarded as a dynamic population equilibrium. In particular the subjects of egg cannibalism and fecundity will be considered. This problem is important because egg cannibalism is, under conditions of high population density, an extremely powerful source of mortality. It is complex because the egg numbers are in a continual state of flux, simultaneously increased by ovipostion and decreased by cannibalism. Very little information can be obtained about the involved rates or the egg survivorship from data on the standing crop of eggs. Chapman (1928) suggested that the imagoes eat their own eggs and Park (1934) actually described this behavior. Cannibalism of eggs has been shown to increase with increased adult density by Chapman (1928, 1933), Chapman and Baird (1934), Chapman and Whang (1934), Park (1933), Stanley (1942), Crombie (1944) and Boyce (1946). The increase in the number of eggs recovered per beetle per unit time, an index of fecundity, has been shown to decrease with increased adult density by MacLagan (1932), Park (1933), Crombie (1944) and Boyce (1946). The phenomenon of decreased reproductive rate under conditions of crowding has been observed in widely different groups of animals and various explanations have been offered in the specific cases. A survey of the work on egg cannibalism shows clearly that the number of eggs destroyed is dependent upon the number of eggs present and the number of imagoes. This is, as pointed out by Allee et al. (1949), a simple, straightforward case of predation-the egg being the prey and the active form the predator. Boyce (1946) concluded that while cannibalism is present, it plays a lesser role than the reduction of fecundity by crowding in the decline of the growth rate of a population. This view is opposed to that of Chapman and Whang (1934), who ascribe the major role to egg cannibalism. Birch, Park and Frank (1951) considered both fecundity and cannibalism under conditions of crowding of females by imagoes of their own species and by imagoes of another species. They concluded that there is a difference in the reduction of fecundity which results from crowding by males and by females. The crowding by females directly affects the fecundity, while the effect of crowding by males can be accounted for by cannibalism. In a mixed-sex culture eggs are being added and destroyed constantly. It would of course be most convenient if cannibalism rates could be obtained from cultures containing only males cannibalizing introduced eggs, and then these data applied to the mixed-sex situation. In some of the work it was assumed that this procedure is valid; however, it was suggested by Stanley (1942) and Boyce (1946) that the rate of cannibalism may be higher for females than for males. In such a case, of course, the rate of cannibalism used would underestimate the rate in the mixed-sex cultures and lead to an underestimate of fecundity rates. For the purposes of this discussion it will be convenient to agree on three definitions of terms which must be used repeatedly: Net fecundity rate-the increase in the number of eggs which are observed or recovered per beetle in unit time. Real fecundity rate-the number of eggs produced per beetle in unit time.