An experiment was carried out to investigate the possibility of retarding the maturation process
in Oreochromis niloticus under laboratory conditions by selection to evaluate an applied breeding
programme relevant for the tilapia industry. The breeding base consisted of 35 full sib families of
the Lake Manzala strain, Egypt. On the basis of a standard testing procedure maturation status was
determined by slaughtering 80 individuals half of each family at an age of 136 days. Gonad .
weight, gonadosomatic index GSI and the visual classification of the gonadal development were .
used as indicators for the maturation process. Family selection was carried out separately for each
sex, using GSI for males and a visual assessment of gonadal development for females as selection
trait. Only families with a minimum weight equal to or above the average family means were
considered for selection to avoid the selection of families with high proportion of underdeveloped
offspring which have retarded or insignificant gonadal development. For establishing the next
generation in the selected line, matings were restricted to combinations of female spawners
derived from selected ‘female’ families with males derived from the selected ‘male’ families but
avoiding full-sib matings. To keep the generation interval as short as possible and to ensure a
condensed breeding period, the heaviest fish within each selected families were used as spawners
for the next generation ca. 25% heaviest fish for males and the 50% heaviest fish for females . A .
random sample of two males and four females per family and generation was chosen as spawners
for the control line. The number of families evaluated varied from 25 to 31 for the selection and
control line in the first and from 30 and 32 in the second generation, respectively. After two