The persistence and degradation of endosulfan isomers and their primary degradation product, endosulfan-sulfate, were studied in a clay soil from cotton farms of western Queensland. Endosulfan degradation in relation to soil moisture, temperature, day and night temperature fluctuation, waterlogging and re-application were studied. The results show that the degradation rates of both endosulfan isomers were greatly affected by changes in soil water content and temperature. Under a high water content-high temperature regime the concentration of a-endosulfan in the soil fell rapidly during the first 4 weeks of application, followed by a prolonged period of slower rate of degradation. Alpha endosulfan showed a bi-exponential form of degradation for all water content-temperature experiments except for extremes in both these two factors. In the submerged soils (and also in low-water content, low temperature, non-submerged experiments) no such rapid initial degradation of a-endosulfan was observed, and a single first-order rate equation describes the data. Degradation of ߭endosulfan was significantly slower than for the a-isomer under all conditions studied. A half-life of more than a year was recorded for the ߭isomer when both water content and temperature were low. The degradation of ߭endosulfan showed no sign of the bi-exponential function observed for a-isomer, and a single first order rate equation described the data obtained for each factor studied. Endosulfan-sulfate was the major degradation product in all non-submerged experiments. Its build up in the soil very closely followed the disappearance of a-endosulfan. Its highest build-up was in the high water content-low temperature experiments, but its persistence was primarily influenced by soil temperature. Both a and ߭isomers, and endosulfan sulfate, persisted longer in the submerged soil. Re-application of endosulfan, and day and night fluctuation of temperature had contrasting effects on the degradation of the two isomers. Both factors slowed down the degradation of a-endosulfan and enhanced that of ߭endosulfan, but their net effect was to prolong the overall persistence of this chemical in the soil. Submerged conditions reduced the net formation of endosulfan-sulfate and enhanced its degradation rate.