A cross-sectional serological survey of dairy cattle in Taranaki in 1979–80 indicated that 62% (551/891) of the animals had evidence ofLeptospira interrogansserovarhardjoinfection as disclosed by the microscopic agglutination test. Titres toLeptospira interrogansserovarpontonawere demonstrated in only 4% (23/591) of the animals examined. The high prevalence ofhardjoinfection is suggestive of an endemic infection whilst the low level topontonais indicative of sporadic infection. In a detailed examination of 10 herds, 9 revealed high (55%–91%) prevalence of serological reactions to hardjo and the herd profiles of titres, indicated that the animals had become infected at one to two years of age. A field strain ofhardjofrom cattle as well as the usual laboratory strain(hardjoprajitno)was incorporated in the test but there were no significant differences between the results given by the two antigens.