The fate of fenthion was examined in microcosms to define the possible interaction between sediment and biodegradation in the field. A mathematical model was also calibrated to calculate the distribution of fenthion in microcosms. Intact sediment cores, both with and without Juncus roemerianus, were removed from a salt marsh and placed into microcosm vessels to simulate the undisturbed sediment bed of a salt marsh and the areas containing Juncus. In formalin-sterilized microcosms without plants, fenthion disappeared exponentially from the water column with a half-life of 105.0 h. Fenthion had a half-life of 35.5 h in the microcosm without plants. In microcosm with plants, the half-life was slightly faster (33.2 h). In fractionated sediment cores, fenthion was found at greater depths in nonsterile systems than predicted by diffusion and sorption in sterile microcosms, possibly because of bioturbation.