Variability in hexythiazox (Savey) toxicity to the eggs of spider mites (Tetranychus spp.) from six laboratory colonies and 26 field collections (5-6 each from cotton, almonds, pears, roses, and strawberries) was determined using a leaf disk bioassay method. The difference in the response of T. urticae Koch and T. pacificus McGregor to hexythiazox was 10-fold when contact and residual methods were compared. Intraspecies differences in susceptibility were found with laboratory colonies of both T. urticae and T. pacificus , indicating that a low level of cross-resistance to other compounds may exist. Variation in susceptibility of fjeld-collected spider mites to hexythiazox within and between crops was also found for both species. Field collected T. urticae were significantly more susceptible to hexythiazox than were field-collected T. pacificus . Few significant differences were found when responses of field-collected spider mites tested on their original host plant leaves were compared with responseson cotton cotyledons in the laboratory. The use of laboratory-reared cotton had far more advantages than the use of host leaves. These results suggest that continued monitoring of hexythiazox susceptibility, determination of cross-resistance patterns, and the implementation of resistance management tactics are warranted before and after this compound is registered for use.