The structure of the dorsal spine in deep sea squaloid shark Centrophorus acus was studied macroscopically and microscopically with relation to its usefulness as an age determination tool. The dorsal spines consisted of five structural components; the mantle, the external anterior dentine portion, the stem, the pulp cavity and the cartilaginous tissue. Growth rings were present in the inner, middle and outer dentines of the stem. However, the number of rings in the inner dentine was greater than that in the middle and outer dentines. The spine length and the number of rings in the inner dentine increased with the body growth. The periodic formation of the rings is discussed. The von Bertalanffy growth curve was fitted by non-linear least squares method. The growth equation is as follows
male: TL_n=1172(1-e^{-0.173(n+1.403}))
female: TL_n=1262(1-e^{-0.155(n+0.485}))
where TL_n indicates the total length at n rings, and n the number of rings.