when matings were conducted to produce the sixth and seventh generations. The sixth generation was sent to Malaysia and was used there to establish the population now located in Jitra Aquaculture Extension Center, Kedah state. That means in between these nine generations, there are two in which no selection of superior individuals for growth rate or any other trait was conducted. If allowance were made for this when calculating genetic gain per generation, a figure (9%) closer to that reported by Eknath et al. (1998) would be obtained.