Studies were conducted in the San Joaquin Valley of California to evaluate the rate and timing responses of Pima cotton (Gossypium barbadense) to mepiquat chloride (PIX). Preliminary data collected in 1991 and 1992 indicate that applications made at early bloom are not economical and can occasionally result in a yield decline. Late-season applications, however, tended to have more positive yield results. Best overall yields came from mid- and late-bloom sequential applications of PIX at a rate 0.025 kg a.i. ha-1, applied at 14 days following first bloom and again at 10 to 14 days following the first application. Fourteen independent studies, occurring over a 4-year period, showed an average yield response that exceeded 60 kg ha-1, although, in most cases, these effects were not statistically significant (P = 0.05).