Oestrosis is a parasitic disease of sheep and goats caused by the nasal bot fly Oestrus ovis. In the United Kingdom the economic losses as a result of infestation can be considered negligible, but the differentiation of O ovis cases from more serious diseases such as listeriosis, gid and sheep scab is of considerable importance. Currently, diagnosis of oestrosis relies on the subjective observation of clinical signs or the demonstration of larvae postmortem. This paper assesses the effectiveness of a direct enzyme-linked immunosorbent assay (ELISA) using a crude somatic antigen from first-stage larvae (L1) in the serodiagnosis of oestrosis. The system has been validated with sera from both endemic and non-endemic areas and the results correlated with the clinical data found postmortem. The sensitivity and specificity of the assay were 97.4 per cent and 97.6 per cent, respectively, using a cut-off point based on 35 per cent binding of a reference positive control serum.