Kelthane MF showed a positive temperature coefficient of toxicity to the two-spotted spider mite, Tetranychus telarius. An increase of about 1.2% mortality occurred for each degree rise in the range from 50-90°F. Pre-treatment of adult, female mites at three different temperatures for three days caused no significant difference in mortality at 74°F. Acaricidal potency of Kelthane MF on peach leaves declined rapidly both indoors and outdoors for the first two days. Thereafter the potency decreased at about the same rate outside, but much slower in the laboratory. Deposits on peach leaves repelled mites strongly for two days, but weakened rapidly over the next six days. Mites entered treated leaf areas after the acaricidal potency had dropped low, and so a larger than expected number of susceptible mites may have survived.