The study of bark structural diversity of 17 gymnosperms was performed to describe it macro and microscopically, to elaborate a key and to compare them with den Outer's classification (1967). External bark allows to identify several species in the field. New anatomical features not reported previously were found, e.g. crystals in cellular walls in Araucariaceae and Taxodiaceae, lack of fibers in Cupressus guadalupensis and lignified sieve cells in the collapsed phloem of Pinus species. Ten species, described for the first time, were assigned to one of the types proposed by den Outer. Although the phylogenetic ideas established for the secondary phloem specialization suggested by den Outer have been questioned by the recent studies on fossil gymnosperms and cladistic analysis.