A programme to assess the cetacean by-catch in the Irish and UK set gillnet fisheries in the Celtic Sea was conducted from August 1992 to March 1994 using volunteer observers. Observers were present for the hauling of over 2500 km of net which caught 43 harbour porpoises and four common dolphins, with one of each alive. The by-catch rate was 7.7 porpoises per 10 000 km · h of net immersion. A negative relationship was found between porpoise by-catch and tidal speed but no other relationships were found with operational or environmental variables. Spatial and temporal stratification of the by-catch rate and effort data had a small effect on estimated total by-catch, which was therefore estimated from pooled data. The estimated total annual by-catch of 2200 porpoises (95% C.I. 900–3500) is 6.2% of the estimated number of porpoises in the Celtic Sea and there is serious cause for concern about the ability of the population to which they belong to sustain this level of by-catch.