Leptospires were isolated from 21.2% of Rattus norvegicus (the brown rat) and 9.9% of Rattus rattus trapped in a survey of urban and rural areas in Israel. Microagglutination antibodies were found in sera of 9.4 and 8.1% of these rats, respectively. The 191 strains isolated belonged to six serogroups: Leptospira icterohaemorrhagiae (169), L. bataviae (3), L. semeranga (1), L andamana (1) and L. ballum (1) from R. norvegicus and L. ballum (13) and L. hebdomadis (3) from R. rattus. No significant differences were found between the infection rate of mature male and female brown rats, but all six strains isolated from young animals were obtained from male rats. Two factors influence the rat of Leptospira infection among urban brown rats: the age of the animals and the area in which they are trapped. Adult brown rats trapped in a newly infected area showed a considerably higher level of contamination (61.8%) than those trapped in areas where the infection had persisted for many years (22.8%). Young rats showed a low degree of contamination in both the old foci (5.7%) and the new foci (2.2%). New and highly infected areas were found during this survey in Tel Aviv (Carmel market) and its neighboring cities of Holon, Ramat Gan and Givatayim.