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Aspects of the reproductive biology of gag Mycteroperca microlepis in the southern Gulf of Mexico were studied by following seasonal variations in the gonado-somatic index and through histological examination of gonads. Gag were collected from inshore and offshore waters of the Campeche Bank, Yucatan, Mexico, between April 1996 and December 2001. This species is a protogynous hermaphrodite, and appeared to be depth-size distributed. The smallest gag (9–49 cm LF) collected were all juvenile females, and were caught in inshore waters (1–10 m depth), while the largest (49–116 cm LF), mainly adult females, males and transitionals, were captured in offshore waters (33–167 m depth). Overall the offshore male to female ratio was female-biased (1 : 3·3) and differed significantly from unity. The species spawns at depths of c. 50–53 m, from early winter to mid-spring, with peak spawning activity occurring between January and March. Fifty per cent of females reached first maturity at 72·1 cm LF. At 103 cm LF, 50% of sampled females had changed into males. Gag can be considered a monandric species, and sexual transition for this grouper seemed to occur in fish distributed within a narrow size range (85–111 cm LF). The results are compared with those of other authors for gag stocks from the south-east Atlantic coast of the U.S.A. and the north-east Gulf of Mexico. [ABSTRACT FROM AUTHOR]