We investigated the relationship between the pH of leaf sap and infestations of whiteflies Bemisia tabaci (Genn.) and Trialeurodes abutilonea (Haldeman) on cotton, Gossynium hirsutum L. in five commercial fields at Maricopa, Az, 1988. Bi-weekly samples of 45 leaves per field were examined for whitefly nymphs and the leaf sap from samples of 10 leaves per field were collected using a pressure bomb. The pressure required for extraction and the pH of each sap sample were recorded. The test period began in June and ended in September or October, depending on final irrigation dates. In general, pressures required for extraction varied little during the season (range ca. 10-15 bars) but rose sharply after final irrigation (range ca. 15-25 bars) as the plants dried out. However, the pH of leaf sap remained very consistent during the entire test period (range ca. 6.3-6.7) including the late season when plants were drying out. The numbers of whitefly nymphs on leaves did not increase during the mid-season period June through August (range ca. 0-1 nymphs per leaf) but increased beginning mid-September and reached seasonal peaks in late September and October (range ca. 15-75 nymphs per leaf). The increased numbers of whitefly nymphs in late season was not correlated with increased pH values of leaf sap as has been suggested in the literature. The reasons for the characteristic late season increase in whitefly infestations in Arizona cotton remains unknown.