<jats:title>Abstract</jats:title><jats:p>The methanol extracts of the roots of <jats:italic>Lonchocarpus cultratus, L. latifolius</jats:italic> and <jats:italic>L. subglaucescens</jats:italic>, species native to Brazil, were all active in the brine shrimp lethality test. Fractions rich in polyhydroxylated alkaloids were obtained from these extracts and, following acetylation, were analysed by GC‐MS leading to the identification of acetylated derivatives of 2,5‐<jats:italic>trans</jats:italic>‐dihydroxymethyl‐3,4‐<jats:italic>trans</jats:italic>‐dihydroxypyrrolidine (1), 1‐deoxymannojirimycin, 1‐deoxynojirimycin, fagomine and homonojirimycin. Only from <jats:italic>L. subglaucescens</jats:italic> could a compound with the constitution and relative configuration expected for the penta‐acetate of 1 be isolated, and its molecular structure was determined for the first time through extensive spectral data analysis. Copyright © 2002 John Wiley & Sons, Ltd.