Abstract: The new agaric species Lactarius moseri Harmaja is described on the basis of Finnish material, and is also reported from Sweden. L. moseri is a close relative of L. vietus (Fr.: Fr.) Fr. In recent years, it has been claimed that L. mammosus Fr. is the correct name for the fungus which is here described as L. moseri. The author, however, suggests that L. mammosus is the correct name for L. fuscus Roll., this conclusion agreeing with the interpretation of many other mycologists. The nomenclature of L. mammosus is clarified; the walls of the cystidia and pseudocystidia of L. mammosus are reported to be amyloid, which is a unique character m the genus.