Two life stages of a laboratory colony of California red scale, Aonidiella aurantii (Maskell) (Hemiptera: Diaspididae), were exposed to 10 concentrations of pyriproxyfen to evaluate the effect of this insect growth regulator (IGR) on scale development and fecundity. First instars exposed to pyriproxyfen responded with mortality during the first and second molts. Second instars were more tolerant of pyriproxyfen than were first instars, indicating that growers should treat California red scale in the field before the first molt to achieve maximum efficacy. Male scales responded with an LC50 12-fold lower and an LC90 47-fold lower than was observed for female scales. Female scales that survived first instar exposure to pyriproxyfen experienced reduced fecundity with increasing pyriproxyfen concentration. Reduced fecundity was due to reduced survival of the females rather than sterility. Results from this study provide baseline California red scale susceptibility data for pyriproxyfen, and they suggest that 1 ppm pyriproxyfen can be used as a concentration that would discriminate between susceptible and resistant populations of scales. Field monitoring for incipient pyriproxyfen resistance in California red scale should be implemented in the San Joaquin Valley of California, and if resistance is detected, integrated resistance management strategies should be used to slow the progression of resistance.