ABSTRACT: The conodont species Streptognathodus simulator Ellison 1941 has been proposed to define the Kasimovian-Gzhelian boundary of the global chronostratigraphic scale based in part on its global distribution (Heckel et al. 2005). Traditionally St. simulator has been used as a marker of the base of the Gzhelian Stage in the type sections in Moscow Basin and Urals, but the proposed definition places the FAD of St. simulator above its traditional lithostratigraphic boundary. This allows the stage boundary to be moved from a position where it is corresponds to a sequence boundary to where it is at or close to a maximum flooding surface. This new boundary appears within the range of Rauserites rossicus (sensu lato) and therefore affects regional and interregional fusulinid biostratigraphy and correlation. Also, the proposed boundary appears above the FAD of fusulinid genus Rugosofusulina. Fusulinid and conodont biostratigraphy within theKasimovian-Gzhelian transition inMoscow, S. Urals and Donets Basin is reviewed and new fusulinid species that allow recognizing newly defined boundary are described.