The content of tryptamine derivatives in Psilocybe semilanceata, a popular hallucinogenic mushroom, was measured by high-performance liquid chromatography. Most of the 52 samples have been collected at several localities in Switzerland during a 1-5 year period. The content of psilocybin and baeocystin varied in the range of 0.21-2.02% and 0.05-0.77%, respectively, whereas only traces of psilocin were present. The variability of the alkaloid level depending on origin, year of collection, size and part of mushrooms is discussed.