The increased importance which the European red mite (Paratetranychus pilosus (Can. et Fanz.)) (= Metatetranychus ulmi (Koch)) has assumed in recent years has led to an intensive study of its biology and natural history. In the course of these investigations many workers, and in particular those in Nova Scotia (vide Lord, 1949), have become convinced that this pest can be controlled, on apple trees at least, by natural means and that some of the most active agents in its eradication are the representatives of that group of predaceous mites which Vitzthum (1941) placed in the subfamily Phytoseiinae Ber'lese, 1916 1). As the late Dr. A. C. Oudemans of Arnhem included many if not most of these species in the genus Typhlodromus as he conceived it, this paper is in essence a revision of that genus. Presumably because of their small size and limited distribution, which is largely contingent upon readily available populations of their hosts, little attention has been paid to these predators from either the ecological or taxonomic point of view. A cursory survey of the literature pertaining to the predaceous relationship which exists between the Phytoseiinae herein to be discussed and the tetranychid mites may serve as an appraisal of this economically significant group of mites. Koch (1839) in describing what now appears to be a typhlodromid, viz., Gamasus vepallidus, made no reference to its possible predaceous habits. Scheuten (1857) thought that the eriophyids which he found associated in numbers with his Typhlodromus pyri were its offspring. Berlese (1882-1898), however, had a better understanding of these relationships and was able to state in his redescription of G. vepallidus as Seius (Seiulus) vepallidus (K.) that it was a predator of small acari as well as being a mycophage. His countryman, Ribaga (1902), writing of the