From the flowers of Arnica chamissonis Less, sub-sp. foliosa (Nutt.) Maguire and its variety incana (Gray) Hulten two new naturally occuring pseudoguaianolides were isolated and their structures established by spectroscopic methods. They are shown to be 4-O-acetyl-6-desoxychamissonolide ( 3) and the propionylic ester of 11,13-dihydrohelenalin which was named arnicolide G ( 6).