This paper investigates determinants of geographic distribution of share tenancy and analyzes its efficiency implications in pre-war Iwate prefecture, Japan. The distribution of share tenancy was attributable to risk represented by yield variability, which in turn was affected by seasonal winds called Yamase and topographic features. That risk raised transaction costs of adopting a fixed-rent tenancy associated with the common custom of rent reduction in Japan that mitigated the problem of risksharing. Estimation results suggest that risk, wealth, and strength of community ties were the main determinants of contract choice.