The chemical constituents of sheep dip in the UK are currently changing from organophosphate-based to synthetic pyrethroid-based insecticides. As a result, changes are also being made to the methods of disposal of these chemicals in the environment, such that pyrethroid sheep dips must now be diluted in animal slurry or water. To date, there is a lack of quantitative information on the impact of the insecticide on the indigenous microflora of animal slurries. This paper investigated the impact of Bayticol (synthetic pyrethroid sheep dip) over a range of concentrations on selected populations of bacteria within animal slurry. It was found that, with increasing pesticide concentration, there was up to a four orders of magnitude increase in the numbers of faecal coliforms and pathogens, such as putative Salmonella spp. These findings have implications for the disposal of sheep dip-amended animal slurries to land from several aspects: (i) the longevity of putative pathogens in the field may require re-evaluation of the time required before the return of grazing livestock to a slurry-amended field; (ii) the potential for the transfer of pathogenic bacteria and faecal coliforms into human and animal foodchains, and (iii) the increased potential for faecal coliforms being washed into streams, rivers and coastal bathing waters.