A serologic survey for leptospirosis in stray dogs from Moreno, Province of Buenos Aires, was carried out. Sera from 143 randomly collected animals were examined by the microscopic agglutination test. A total of 73 (51.0%) of the dogs have had contact with leptospirae. The predominant leptospiral agglutinnins were to serovar canicola (37.0%) and to a lesser degree against the serovar antigens pyrogenes (4.8%) and autumnalis (2.0%). Although a high percentage of seropositive reactions occurred against the serovar antigens ballum, pyrogenes and icterohaemorrhagiae, all the reactions against ballum and icterohaemorrhagiae, and the majority of the reactions to pyrogenes represented the natural coagglutinins frequently associated with serovar canicola. The detection of antibodies to the Autumnalis group in dogs of Argentina could be new and suggest the emergence of canine infections caused by a serovar of this serogroup which should be sought out and identified.