Ivermectin (22,23-dihydroavermectin B1) is a recently discovered, persistent, broad-spectrum, antiparasitic drug of unpredecented potency which is now routinely administered to cattle, horses, sheep and pigs in many countries. In cattle, it is an efficient control for parasitic gastrointestinal and respiratory tract nematodes, warble fly, mites, lice and ticks. However, most of the ivermectin dose is ultimately eliminated in the faeces of the treated animals where it has been shown to have an insecticidal effect on the larvae of economically important, dung-breeding, haematophagous Diptera. Nevertheless, the effects of excreted ivermectin on the cowpat fauna as a whole and the wider consequences of such effects have not previously been considered. In field trials reported here, the faeces of calves fitted with rumenal boluses delivering ivermectin at 40 micrograms per kg per day, failed to degrade in the normal way and this failure was associated with the absence of dung-degrading insects. Faeces from placebo-treated controls contained a characteristic dung-degrading invertebrate community and were largely degraded within 100 days. These results indicate that the increasing widespread use of ivermectin may have important environmental consequences for pastureland.