Tricyclohexyl tin hydroxide (TCTH) was found to be relatively toxic to the citrus red mite, Panonychus citri (McGregor), and the Pacific spider mite, Tetranychus pacificus McGregor. It was slightly more toxic to the nymphs than to the adult females and more toxic by direct contact than as a spray residue. There was no change in the susceptibility of our laboratory strain of the Pacific spider mite after 10 selections with TCTH where selection occurred at the LC50 level. Residues from field-spray applications to lemons were toxic to adult mites for 30–60 days after applications. The time interval of toxicity was dependent on the dosage and time of year applied. During an elapsed period of more than 20 days a sizeable proportion of TCTH residues could be removed from lemon fruit and leaves by a detergent wash as indicated by the toxicity of the residues to adult female mites. Effective control of the citrus red mite was achieved in more than 20 experimental orchards by dilute or “misttype” spray applications. However, under some conditions TCTH sprays were slightly less effective than the quinoxiline applications which were used as a standard treatment. Necrotic spots were observed on very young citrus growth following some spray applications. Severe leaf drop occurred when TCTH was applied to lemon trees immediately following an application of 1.2% petroleum oil.