<jats:title>Abstract</jats:title><jats:p>A spray reagent which is sensitive and specific for polyhydroxyindolizidine glycosidase inhibitors such as swainsonine and castanospermine has been developed for their visualization on thin layer chromatographic plates. The method involves treatment of the plates with a sequential system consisting of 10% acetic anhydride in benzene, followed by Ehrlich's reagent. This technique has been used to follow the isolation and purification of minor alkaloidal constituents from <jats:italic>Astragalus lentiginosus</jats:italic> and <jats:italic>Castanospermum australe.</jats:italic> The occurrence of swainsonine and its N‐oxide in a number of <jats:italic>Astragalus</jats:italic> and <jats:italic>Oxytropis</jats:italic> species has been investigated. Species previously incriminated as locoweeds on the basis of field observations were all shown to contain the alkaloid and it was also found to occur in additional species. The highest levels of swainsonine were found in the seeds and pods, with lesser amounts in the leaves and stems, and none detectable in the roots.