THE YUMA SPIDER MITE, Eotetranychus yumensis (McGregor), was first observed near Yuma, Arizona, in about 1928 by J. L. E. Lauderdale, and was described in 1934 by E. A. McGregor from specimens collected on lemon foliage by R. S. Woglum and H. C. Lewis. This mite occasionally becomes a serious pest on citrus in certain desert regions of the southwestern United States, but little information has been published on its life history, distribution, host range, and the means of reducing injurious populations.