The swede midge Contarinia nasturtii (Kieffer), a member of the family Cecidomyidae (Order, Diptera), is a small insect, the larvae of which induce gall formation on numerous cruciferous plants. Kieffer (1888) first described the midge which he reared from larvae occupying the closed flowers of Nasturtium palustre D. C., now known to be Rorippa islandica (Oeder) Borbas. In subsequent years numerous authors have reported the midge, under various synonyms, as a frequent pest of several important cruciferous crops. Briefly this thesis re-examines the life-cycle of the swede midge in detail and investigates the effect of what seem to be the important environmental factors on successive stages of development. After consideration of the observed population fluctuations of C. nasturtii at Nafferton, Northumberland, during 1957-1960, the thesis concludes with a discussion of population change in relation to various components of the environment.