Fumigant pesticides are frequently used in intensive agriculture to control nematodes, fungi, and weeds. Currently, four registered fumigants are available: 1,3-dichloropropene (1,3-D), methyl isothiocyanate (MITC), chloropicrin (CP), and methyl bromide (MeBr). The use of 1,3-D, MITC, and CP can be expected to increase after MeBr is completely phased out of production in the USA in 2005. In soil, the degradation of 1,3-D, MITC, CP, and MeBr occurs through both chemical and biological mechanisms. Repeated applications of the fumigants MITC and 1,3-D are known to enhance their biodegradation as a result of adapted microorganisms. Preliminary evidence suggests that the microorganisms responsible for enhanced degradation of MITC specifically target the isothiocyanate functional group. In the case of 1,3-D, a number of bacteria have been isolated that are capable of degrading 1,3-D and also using it as a sole C and energy source. Of the two isomers of 1,3-D, degradation of trans-1,3-D was found to be greater than that of cis-1,3-D in enhanced soil. Methyl bromide is mainly degraded chemically in soil by hydrolysis and methylation of nucleophilic sites on soil organic matter. Both degradation reactions occur via S2N nucleophilic substitution. Methanotrophic and ammonia-oxidizing bacteria can co-oxidize MeBr during the oxidation of methane and ammonia, respectively. The microbiological degradation of MeBr is apparently catalyzed by methane and ammonia monooxygenase. Chloropicrin can be dehalogenated by Pseudomonas spp., with the major metabolic pathway occurring through three successive reductive dehalogenations to nitromethane.