Naumannite (Ag₂Se) is described from a mine - Johannes Gruve - in the Kongsberg silver district of south Norway. Physical data and chemical analysis are given. Single crystal work decides an orthorhombic symmetry, and the cell parameters and a list of indexed d-values are presented. The mineral shows invertion twins. The naumannite-chalcopyrite-quartz-calcite paragenesis is formed by fracture filling.