In a search for leptospirosis in Belize, at least one current case was diagnosed among 11 febrile hospital patients examined in a 2-week period in mid-1984. Sera from 440 survey subjects in 6 schools, 4 urban communities and 9 rural communities were examined for leptospiral antibodies by the microscopic agglutination test. 20 of 174 (11.5%) school pupils, 19 of 88 (22%) urban community subjects and 66/178 (37%) rural community subjects were positive at titres greater than or equal to 1:50, and 7.5%, 17% and 26%, respectively, were positive at greater than or equal to 1:100. Among all groups of survey subjects, serological reactions to antigens in the Australis serogroup predominated. Up to 81% of 155 sera from cattle in various areas of Belize were positive at titres greater than or equal to 1:100. Serological reactions to serogroup Sejroe predominated (43%), followed by Pyrogenes (16%) and Australis (15%). 20 of 71 (28%) pig sera were positive at greater than or equal to 1:100, predominantly to serovars in the Australis serogroup (51%). Of 162 sheep or goat sera examined, 65 (40%) were positive at greater than or equal to 1:100, mainly to serogroup Autumnalis (25%), followed by Australis (15%).