The caudal skeleton is recognized as an important source of information for establishment of teleostean interrelationships. A comparative analysis of the caudal skeleton of Achirus lineatus (Linnaeus, 1758) and A. mazatlanus (Steindachner, 1869) was performed, based on the examination of specimens from biological museums and others materials collected in Veracruz localities and Baja California Sur (Mexico). The specimens were identified using taxonomic keys and processed by means of clearing and staining methods, in order to determine the differences and similarities on the elements that characterized the caudal complex of this species. The caudal formula in A. lineatus and A. mazatlanus is composed by: 1 epural + 5 hypurals + 1 parhypural, one neural and two hemal spines, as well as the presence of 16 principal rays and absence of procurrent rays and the uroneural bone. The hypural in the A. lineatus are closer together; however, in general, differences were not observed among the species. Therefore, A. lineatus and A. mazatlanus are established as sister species.