A total of 431 rats, collected twice monthly during one year from the same locality (a slum-commercial area of approximately 1 km2 in the southern part of Tel-Aviv), were examined for the presence of S. ratti and S. venezuelensis. Of the 431 rats examined, 154 (35.7%) were found infected as follows: 48 with S. ratti, 37 with S. venezuelensis and 69 with both species. A seasonal fall in infection, from a high incidence of 78·9% in February to a low incidence of 4·5% in September was noted, which roughly corresponded to the monthly rainfall. The persistence of infestation throughout the rainless summer months is probably due to seepage from surface structures (warehouse depots, dripping outdoor faucets, faulty plumbing, etc.). The monthly incidence of both species of parasite according to age and sex of the host is discussed.