Reproductive aspects and length-weight relationship were measured for the redbelly tilapia, Coptodon zillii in the Shadegan International Wetland, southwest of Iran. A total of 1161 specimens (520 males and 641 females) were collected from the wetland during the period from September 2015 to August 2016. The fish mean length ranged from 5.2- 24cm while mean weight ranged from 11-265g. The species indicated a positive isometric growth pattern with a ‘b’ value of 2.90 and 2.95 for the males and females, respectively. The overall sex-ratio (M/F) was 1:0.81. Adult females had an average of 180.5 eggs per cm of length. The result of Gonadosomatic index analysis showed that the period from April to September may be representing the spawning period of C. zillii. The long period of spawning shows an effective factor in the reproduction of C. zillii in the wetland, causing a high abundance of the species.