Puerto Rico, which has been considered one of the world''s rabies-free areas, is experiencing an outbreak of rabies. Until the present outbreak, no case of rabies had been reported on the island since 1933. The chain of events with regard to the occurrence and spread of this infection established it as primarily an epizootic of rabies in the mongoose population with secondary transmission to dogs and other domestic animals. This is the first major outbreak of rabies in the Western Hemisphere attributed to the Indian mongoose, Herpestes javanicus. From the onset of the outbreak on March 22, 1950, to Sept. 18, 1951, 78 cases of rabies were confirmed by laboratory examination. These included 37 cases in mongooses, 18 cases in dogs and 23 cases in cats and livestock. 104 treatments of Semple human antirabic vaccination have been administered by the Health Department. Immediately after the outbreak, emergency control measures were put into effect to prevent the spread of the disease in the large and susceptible dog population. Plans were drawn to control the disease in the mongooses of the island by a mongoose trapping program.