This study aimed to describe updated information on the basic reproductive biology of Sphoeroides annulatus females and potential use of the gonadosomatic index I G ð Þ as input data to assess length at 50% maturity (L 50 ) and identify the breeding season. FromApril 2014 to December 2015, 767 females were analysed along the north-west coast of the Mexican Pacific Ocean. Both immature and mature females were represented in the total length (L T ), which ranged from 81 to 440 mm, and the smallest mature female was 92 mm (new record for the species). Gonadal maturity phases were assigned through histological analysis. A I G threshold value I cut