In previous investigations on small mammals in Finland} (Rislakki & al 1954, Rislakki & Salminen 1955, Salminen 1956), the Leptospira icterohaemorrhagiae-frequency in rats (Rattus norvegicus) was rather high (43.1 %). Leptospira-positive cases were also found in house mice (L. sejroe 23.3 %), harvest mice (L. hataviae 9.0 %), yellow necked field mice (L. poi 12.5 %), common voles (L. sejroe and L. bataviae together 12.1 %), field voles (L. sejroe and L. bataviae together 10.7 %) and in common shrews (L. poi 1.2 %). Specimens of other species sent in for investigation (Norway rat, common red backed vole, large tooth backed vole, northern red backed vole, root vole, water vole, wood lemming, Laxmann's shrew, lesser shrew and water shrew) gave negative results.