The age and growth of the scalloped hammerhead shark, Sphyrna lewini (Griffith and Smith, 1834) were estimated from rings on the vertebral centrum in 101 individuals (51 females and 50 males) from 260 organisms obtained from commercial fisheries off the coast of Michoacán, Mexico (18º10'N, 103º10'W). Results show that immediately after birth, a first ring is formed. Later we observed that two rings are formed each year, one between June and July and the other between November and December. The parameters of the von Bertalanffy growth function were, for females, L∞ = 353.3 cm, K = 0.156 cm yr–1, t0 = –0.633 years, b = 2.8 and W∞ = 269.92 kg; and, for males, L∞ = 336.4 cm, K = 0.131 cm yr–1, t0 = –1.09 years, b = 2.87 and W∞ = 187 kg. According to these results, a female 335.6- cm-long would be 18.6 years old and a male of 244.3 cm would be 8.8 years old.
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