A method was described whereby information was obtained from field populations of the cabbage aphid, Brevicoryne brassicae and the results were used for the development of life tables. Statistical analyses of the life tables to find the key factor which could control population numbers from generation to generation revealed that the adult stage contained the key factor in the populations which developed from apterous parents (whether this factor was parasitism, brought about by D. rapae, or a mortality induced by a factor which caused the aphid to fall off the host plant could not be ascertained) and the first nymphal instar contained the key factor in the populations which developed from alate parents,. However, as only a limited number of large populations were studied and as the lengths of the nymphal instars had not been recorded during the experiments, these results could not be accepted as valid. The results obtained were, however, used to develop a sampling plan which could be used for a future study and to obtain information on the biology of the aphid under field conditions. The rate of development was shown to be related to the mean temperature for development and the daily mortality rate. Daily fecundity appeared to be independent of both daily and developmental temperatures and age of the parent aphid. Therefore although the results of this study were not sufficient for a population dynamics study, it has provided the basic material for developing a sampling plan, shown where more information would be needed in a later, more detailed study and illustrated that it is possible to overcome the problem of the overlapping of the generations in aphid populations so that quantitative ecological information can be obtained for the development of population dynamics models.