Less than one per cent of serum samples taken from 257 fish farmers in 1981 had agglutinating antibodies to strains of Leptospira interrogans of serogroup Icterohaemorrhagiae at a titre of 30 or greater. Compared with the results from other serological surveys, this agglutinating antibody prevalence suggests that fish farming does not have a high occupational risk for leptospirosis. Between 1961 and 1981 the incidence in fish farmers was about 33 per 100000 person years at risk. During the same period the incidence in the general adult male population was 0·137 per 100000 person years at risk, so that fish farming had a moderately increased risk of Icterohaemorrhagiae serogroup infection (relative risk = 243). No one particular risk factor within fish farming could be reliably identified and therefore recommendations to reduce the risk can only be general.