The work described is a study of the selectivity of three aphicides. The effects of phorate, menazon and dimethoate on Aphis fabae, Myzus persicae and Acyrthosiphon pisum, feeding on beans (Viccia faba), are compared with their effects on two aphid predators. These were Anthocoris nemorum, which is associated with a variety of plants and prey, and Anthocoris confusus, which occurs mainly on trees, where it feeds on aphids. Aphid bioassays are described, which establish the effects of the aphicides, applied to the cut roots of beans in the laboratory, thus providing LD50 data for comparison with dosages affecting Anthocorids. The effects of the aphicides on aphids are compared with their different effects on Anthocorid eggs laid in the same plants. Field Experiments were also done with phorate and menazon, applied to soil at different concentrations, to assess the effects of normal agricultural treatments on Anthocorid eggs laid in bean plants. Differences between egg mortality of A. nemorum and A. confusus, were compared with the different oviposition sites of the two species. The distribution of phorate derivatives, from root applications, in beans and other crop plants, was studied, using P32 labelled phorate, and this is compared with the oviposition site of A. nemorum in these plants. The action of dried deposits of the aphicides, on Anthocorids and aphids, is compared with the action via roots. The uptake of phorate derivatives by Anthocorids, from bean plants and poisoned aphids was demonstrated, using S35 labelled phorate applied to the roots. The effects of the aphicides, thus recieved, on survival and fecundity of Anthocorids is described. The different effects of the aphicides, on all stages of Anthocorids, is discussed, with particular emphasis on the effect of phorate on Anthocorid eggs.