Yellowfin tuna, Thunnus albacores, occur world-wide in tropical and subtropical oceans and support major fisheries throughout its range. In the Pacific Ocean, the species is known to spawn every 1.1 - 1.5 days during a four to six month reproductive season (McPherson 1991, Schaefer 1996); the four month reproductive season in the Indian Ocean appears related to the north monsoon (Stéquert et al. 2001). However, although yellowfin tuna are known to frequent the Gulf of Mexico (GOM) (Weng et al. 2009) and support an active recreational fishery, there is no information on the reproductive biology of the species in the region. This study provides the first information on spawning seasonality, sexual maturity, and gonadal development of yellowfin tuna in the northern GOM.