Results from field experiments in New York establish realistic yield levels for the Three Sisters, a polycultural cropping system of inter-planted corn, bean, and squash which was used by Iroquoian farmers in the 16^th, 17^th, and 18^th centuries. A traditional open-pollinated white flour corn yielded from 22 to 76 bu/acre (1155 to 4127 kg/ha); the higher yields were obtained in New York's Lake Plain region with its fertile soils and long growing season. Corn grain yields were also higher at increased plant densities and closer spacing. We argue that these grain yields, which are significantly higher than other scholars have reported, are an accurate reflection of the yield potential of the productive soils of this region. Other researchers have failed to account for the complex and critical ways that soil organic matter and agronomic practices determine corn yields. Our yield estimates are further supported by eyewitness accounts that describe a highly productive agriculture practiced by Iroquoian farmers in the 16^th through 18^th centuries. Using Land Equivalency Ratios, we also demonstrate that the three crops grown as a polyculture were more productive than monocultures of individual crops. Corn grain yields were little affected by competition from bean and squash, but these two crops yielded much less in the polyculture compared to monocultures. We suggest that the Three Sisters, while widespread, was not universal across Iroquoia.