A project evaluating the performance of cv. Hass on eight clonal avocado rootstocks—G755A, G755B, G755C, Duke 7 (D7), Borchard (BR), D9, Toro Canyon, and Topa Topa was established in southern California in 1986. Two additional rootstocks, Thomas and G1033, were added in 1987. Of the trees planted in 1986, the BR and D7 rootstocks have consistently had the highest total yields for all rootstocks, whereas the three G755 selections have had the lowest productivity. No differences in productivity between the two rootstocks planted in 1987 have been detected. The influence of rootstock on the magnitude of alternate bearing will be discussed, although the oscillation in yield is greater for the higher-yielding rootstocks. Tree size has been measured throughout the study. The BR selection has consistently produced a larger tree, even though it has continued to have high productivity. There are no consistent differences between the other rootstocks. Yield efficiency, measured as the kg fruit/m3 of canopy volume has been calculated. In selections that are prone to severe alternate bearing, the swing in yield efficiency is also the greatest. The data thus far suggests that a yield efficiency of ≈2.5 kg fruit/m3 canopy volume is the maximum yield possible for California `Hass' avocado.