AbstractA new exposure index is described which can be derived from wind velocity, direction, duration and the effective fetch. Effective fetch is calculated from actual fetch modified by shoreline and offshore extents of shaIlow water. The index is calculated from the sum of these components for each unobstructed 22.5 degree compass sector of the shoreline. Data are presented for tests of the index as a predictor for littoral zonation phenomena in Bermuda and the Bayof Fundy, Canada. For the zones in both locations the index satisfies the majority of variance in the data. The vertical distribution of species is weIl modeIled in many cases, but not for others. In many cases significantly better fits are obtained if the basic index is modified using a component for shore slope. The index is useful as a numerical index of exposure which can be derived from standard weather data and hydrographic charts. It is also useful in pointing out littoral species that either do not respond to exposure, or respond in a different way from the majority of species.