1 – Rat leprosy hás been found in the city of Rio de Janeiro. Among 10.000 rats examined there were 37 that presented characteristic lesion of rat leprosy disease, out of which 32 were Rattus norvegicus (86,5%), 3 Rattus alexandrinus (8,1%) and 2 Mus musculus (5,4%). 2 – The disease was observed mostly in adult rats, i.e 94,6%; the rest being observed in young ones. 3 – Probably the sex has not influence upon the infection, but the A. has found 24 males and 13 females affected, a ratio of 1.8:1. 4 – The glandular form of the disease was the most prevalent being observed in 25 rats, 9 rats showed the musculo-cutaneous form and 3 the mixed form. This one was noticed only in advanced cases of the infection.