Greater resistance to spider mites is the goal of a breeding program underway with Acala cotton at the U. S. Cotton Research Station, Shaffer. Differences in susceptibility to spider mites, previously observed between genetic cotton types, are being utilized in present efforts to develop a mite-resistant cotton. Spider mite infestations have increased steadily since 1945 and this group of pests (Tetranychus spp.) now ranks as a major problem for cotton growers in California.