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Dissipation rate of prometryn, oxyfluorfen and imazethapyr was estimated in the Mollisol San Anton and the Vertisol Fraternidad soil series, both soils from the south semi-arid coastal region of Puerto Rico. The study was carried out In two consecutive years. The herbicides were applied one day after seeding pigeon peas at Juana Diaz (San Anton soil series) and Lajas (Fraternidad soil series) Agricultural Experiment Substations. Herbicide rates were 2.24, 0.28 and 0.070 kg ai/ha for prometryn, oxyfluorfen and imazethapyr, respectively. Soil samples at 15-cm depth were collected at 0, 28, 56,112 and 168 days after application. Field half-lives between 2 and 43, 31 and 88, and 19 and 46 days were found for prometryn, oxyfluorfen and imazethapyr, respectively. Substantial dissipation rate variability for above mentioned herbicides has been shown to occur within the soil and year treatments. These findings suggest very short and variable persistence for the three herbicides in both soils under a tropical climate.