Populations of phytoseiid mites on apple foliage were studied in a natural (unsprayed) and 2 artificial (captan and DDT-captan) environmental blocks in a “Northwestern Greening” apple orchard in Door Co., Wisconsin, from 1959 through 1962. Twelve species of phytoseiids were found of whichTyphlodromus caudiglansSchuster,Amblyseius fallacis(Garman),Typhlodromus longipilisNesbitt, andAmblyseius zwoelferi(Dosse) were the most common in that order.T. caudiglanswas far the most common species annually in the check and captan blocks andA. fallacisin the DDT-captan. The latter species' tolerance to DDT increased annually.