The fate and behaviour of beta-cypermethrin in soil has been investigated in laboratory studies using 14C-benzyl and 14C-cyclopropyl beta-cypermethrin. The route and rate of degradation of beta-cypermethrin was investigated in four aerobic soils (loamy sand, sandy loam, clay and silt loam) at 50% Maximum Water Holding Capacity and 20oC. The pH of the four soils used ranged from 5.4 to 7.2 when measured in CaCl2 which is acceptable as the rate of degradation is not expected to be pH dependent (on the basis of no pH dependence in the 4 soils tested). The stereoisomeric test substance [14C]beta-cypermethrin consisted of approximately 40% cis-isomers and 60% trans-isomers. Over the study period the trans isomers degraded more rapidly than the cis isomers so that by the end of the study the ratio of cis:trans isomers had reached approximately 1:1. Beta-cypermethrin degraded in aerobic soil via the formation of the two major metabolites, PBA (3-Phenoxybenzoic acid) (max. 17.1% AR) and CPA (cyclopropanecarboxylic acid) (max. 27.5% AR), bound residues (max. 34.0% AR) and 14CO2 (max. 45.1% AR). Negligible organic volatile radioactivity was formed in the soils tested.