This study assesses the reproductive biology of the yellowfin, Thunnus albacares, of the Western Indian Ocean. A total of 1561 yellowfin were sampled from commercial catches on-board of a purse-seiner in three different areas of the Western Indian Ocean and Seychelles cannery (from January 2009 to May 2010).

Besides the description of different oocytes developmental stages, some preliminary results that could be compared with previous studies have been obtained. In the present work the size at first maturity was estimated at 77, 8 cm, the female-ratio was found to be 1: 0.9, mean batch fencundity of 2.5 million oocytes and the mean relative batch fecundity of 61.9 oocytes per gram of body weight was calculated. Moreover, the area between 0º north and 10 º south has been identified as the most active spawning ground and January, February and June have been the months when most developed ovaries were found corresponding the highest GSI values (over 1.5 GSI value).