AbstractRelationships among families of the shark order Carcharhiniformes are still being debated, for example, in relation to the inclusion of hammerhead sharks (Sphyrnidae) and the tiger shark (Galeocerdo cuvier) in the Carcharhinidae. Previous morphological studies have not analyzed the phylogenetic significance of axial caudal muscles among carcharhiniforms. The authors therefore describe here the axial muscles of the tail region (m. epaxialis,m. obliquussuperioris,m. obliquusinferioris,m. flexor caudalis) to better understand their anatomical patterns among carcharhiniform sharks. Our results indicate that the family Pentanchidae, recently separated from scyliorhinids (catsharks), present a similar pattern of caudal muscles to catsharks, indicating their close phylogenetic relationship. Triakids, hemigaleids, leptochariids,Galeocerdo cuvierandPrionace glaucahave a unique pattern in which the medial axial bundle is bulkier than all other caudal axial muscles. Sphyrnids share with carcharhinids a similar pattern of them. obliquussuperioris(approximately the same size as the dorsalm. epaxialis) and have fibers of them. flexor caudalisarranged in a V‐shape. This chevron‐shape is also present on them. flexor caudalisof hemigaleids and leptochariids. Also, sphyrnids and carcharhinids have more slender septa between muscle bundles (more notably in carcharhinids; the only exception isGaleocerdo cuvier, which has the same pattern as sphyrnids). Although being used here as an outgroup, Squaliformes is unique in lacking one of the epaxial bundles. The authors conclude that the caudal axial musculature has greater phylogenetic potential than previously understood.