Psilocybe semilanceata contains hallucinogenic indole alkaloids and has been used as a narcotic drug in Norway. The content of psilocybin, the major constituent in Norwegian P. semilanceata, has been investigated as well as its habitats and its distribution throughout the country. The mushroom is growing on grassy sites in most parts of Norway from the middle of August to the middle of October. The psilocybin content in dried mushrooms varied considerably, from 0.17 to 1.96%. The highest concentrations in percent was found among the smaller mushrooms whereas the content in mg was highest in larger mushrooms. The content of psilocybin makes P. semilanceata to a potent hallucinogenic drug.