AbstractSixteen populations of O. lambertii, collected from Kansas, Oklahoma, Arizona, Utah, Colorado, Nebraska, South Dakota, New Mexico and Wyoming, USA were examined for their swainsonine content and tested for the presence of Alternaria spp. Significant concentrations of swainsonine was recorded in 2 O. lambertii populations each from New Mexico and Arizona, and one from Utah, with concentrations varying from 0.008-0.054%. Populations containing significant levels of swainsonine were restricted to O. lambertii var. bigelovii and geographically found in the most southern and western portion of its distribution. Swainsonine was also recorded from the remaining 11 populations of O. lambertii, although at very low concentrations. The endophyte, Alternaria was cultured from the 5 O. lambertii populations that recorded significant amounts of swainsonine.