The paper examines the recent emergence of a water pollution 'problem' in Britain associated with agricultural pesticides, and addresses the following questions: (i) how have pesticides become such an important part of arable farming practice in Britain; and (ii) how has the 'problem' of pesticide pollution been defined and contested by different groups. Farm survey evidence from the River Ouse catchment will be used to show how farmers decide to use pesticides, how they legitimise and represent their practices; and how they understand the associated environmental risks. The paper concludes that the role of pesticide advisors and the perception of weeds in farming culture remain important barriers to the reduction of pesticide use.