Among 116 stray dogs in Puerto Rico surveyed for leptospiral agglutinins, 73 (62.9%) has significant titers to one or more leptospiral serotypes. The most common serogroup identified serologically was icterohaemorrhagiae accounting for 53 (72.6%) of the infections. Although the infection rates were comparable for male and female dogs the older were found to be more frequently infected. The significance of the canine in the epidemiology of leptospirosis in Puerto Rico is probably of paramount importance due to the extensive direct and indirect contact dogs have with rats and the human population.