Laboratory experiments were conducted to investigate the enhanced degradation of isofenphos [ 1- methylethyl 24 [ethoxy[ (1-methylethyl)amino]phosphinothioyl]oxy] benzoate] in soil and to elucidate the microbiology of this phenomenon. [U-ring-'4CIIsofenphos was most rapidly degraded in Iowa cornfield soils that had a history of isofenphos insecticide use. Between 13 and 42% of an applied dose of 5 ppm remained as isofenphos after 4 weeks in soil with isofenphos use history, whereas between 63 and 75% remained in comparable nonhistory soils. Soils with enhanced isofenphos degradation contained an adapted population of soil microorganisms responsible for the degradation observed. Degradation products of isofenphos detected in cultures of adapted soil microorganisms included isopropyl salicylate, 14C02, and polar products. A bacterial strain (Pseudomonas sp.) isolated from soil with enhanced isofenphos degradation proved capable of utilizing isofenphos as a sole carbon source.