Degradation of carbofuran in a topsoil sample from a previously untreated ΓΌeld site in the UK was characterized by a short lag period followed by rapid degradation. Carbofuran added subsequently to the same soil degraded rapidly without the lag period. In a subsoil sample from the same site, the ΓΌrst application of carbofuran degraded only slowly but degradation rate increased with subsequent treatments and the third dose degraded as rapidly as in the pre-treated topsoil. In similar experiments with ethoprophos, degradation was quite slow and enhanced degradation of subsequent additions of ethoprophos was not observed. A single application of carbofuran in the ΓΌeld in the UK activated soils for rapid biodegradation of the insecticide for at least the subsequent four years. In contrast, in soils from Greece, enhanced degradation was evident six and 18 months after the last carbofuran treatment in the ΓΌeld, but not after three years. Fifty per cent of ethoprophos applied to soils from Greece previously treated with the nematicide was lost within approximately four days, compared with 38 days in a similar, but previously untreated, soil. Very rapid degradation of ethoprophos and carbofuran was observed in soil samples from Greece which had been treated annually with ethoprophos for the last 30 years but with no previous applications of carbofuran. Annual use of the thiocarbamate herbicide EPTC in the same ΓΌeld may have resulted in cross-activation for rapid biodegradation of carbofuran. Very slow degradation of both carbofuran and ethoprophos was observed in soil samples with a history of combined applications of the two pesticides, probably because of their low pH. Fumigation of soil with chloroform, or treatment with the antibacterial antibiotic chloramphenicol, inhibited ethoprophos degradation in a soil where rapid rates of loss had previously been induced, but the antifungal antibiotic cycloheximide had no eΓΎ ect on degradation rate.