The common coactions between plant and animal populations can readily be studied in the normal Drosophila melanogaster culture, and from this laboratory model it may be possible to obtain some indication of the significant variables affecting the growth of similar populations in nature. This model may also be used to test the validity of the assumption inherent in the various mathematical theories of population growth. The present experiments are concerned only with what various circumstances determine oviposition rates and fecundity of the parents. The growth of the population is determined in the first place by changes of this rate. Under optimum conditions the daily egg output of the flies changes as they age. This change in time is described for flies fed with different quantities of food (yeast) and also for flies fed on food of different qualities. Flies fed only on sugars lay a minimum number of eggs, and fecundity increases in proportion to the relative amount of food supplied to the flies. Only a very small amount of yeast is necessary to maintain normal oviposition. The rate of oviposition is also determined by the quality of the yeast supplied to the flies. Baker's yeast produces a higher rate of egg output than brewer's yeast, and live yeast gives a greater output than autoclaved yeast. This difference is due to the amount of specific substances, probably vitamins, in the food. But total fecundity is but little affected by these differences of quality, since flies which lay less rapidly live longer and produce the same total number of eggs as those which lay more rapidly. Total fecundity is approximately proportional to the size of the flies on hatching. However, the quality of food available to them during their larval stage does affect their oviposition rates. In population cultures, rate of egg output is suboptimal after the second to third day. The changes of the quality of yeast in the cultures is probably the significant ecological agency which produces this lowering. After the sixth day, destruction of the medium surface by larval activity and disappearance of yeast from that surface may be of some importance. The course of daily egg output in such cultures must lead to a phase of 'family growth' which might be described by a logistic curve. Crowding of adults leads to only a slight lowering of fecundity when the flies are adequately fed. There is also little evidence of competition for oviposition space within the limits tested. So the decrease of fecundity demonstrated by Pearl (1932) can take place only when there is competition for food and is the direct result of this competition. This is the correct explanation of his results. These findings are discussed in the light of ecological theory.