Suspensions of soils treated in the field with alachlor (2-chloro-2',6'diethyl-N-(methoxymethyl)acetanilide) and propachlor (2-chlor-N-isopropylacetanilide) were tested for their ability to metabolize these herbicides. Less than 8% of (14)C ring-labeled alachlor was mineralized in 30 days at concentrations of 10 and 0.073 micrograms/ml. The soil suspensions mineralized 16-61% and 0.6-63% of ring-labeled propachlor in 30 days at a concentration of 0.025 and 10 micrograms/mL of suspension, respectively. Although soils converted alachlor to organic products, microorganisms able to mineralize the pesticide could not be isolated. Samples from ground water aquifers mineralized less than 1% of the herbicides at the lower concentrations, but four organic products were formed from alachlor. A mixture of two bacteria mineralized 57.6% of ring-labeled propachlor in 52.5 h. A product of the microbial metabolism of propachlor was identified as N-isopropylaniline. These findings suggest that mineralization is a major means for the destruction of propachlor but not for alachlor in soil. (Copyright (c) 1986 American Chemical Society.)