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The catabolic activity of incumbent microorganisms in soil samples of eleven dissimilar soil series was investigated, with respect to the herbicide isoproturon. Soils were collected from a 30×37 km area of river catchment to the north-west of London, England. Catabolic activity in each soil type during a 500 h assay was determined by ¹⁴C-radiorespirometry. Results showed four soils that exhibited high levels of catabolic activity (33–44% mineralisation) while the remaining seven soils showed lower levels of catabolic activity (12–16% mineralisation). There was evidence to suggest that soils exhibiting high catabolic activity had low (<22%) clay content and tended towards lower organic carbon content (<2.7%), but that these higher levels of catabolic activity were also related to pre-exposure to isoproturon. The ¹⁴C-radiorespirometric results were used to produce a GIS layer representing levels of catabolic activity for …