The present study was undertaken to investigate colour and sex determination mechanisms through the application of androgenesis, gynogenesis and controlled breeding programme with the objective of producing all red males in 0. niloticus. The highest yield of androgenetic haploid to pigmentation stage was 24.6±3.5% (relative to controls) with optimal UV irradiation dose of 450Jm"2 for 5 minutes. The highest survival rate of diploid androgens was 0.07±0.07% (relative to controls) to yolk sac stage using a heat shock of 42.5°C for 3 minutes 30 seconds applied at 25 minutes after fertilisation. All paternal inheritance of diploid androgenetic tilapia was verified using DNA fingerprinting. The mean recombination frequency of the red skin colour gene in meiotic gynogens was 0.12±0.04. All maternal inheritance of meiotic gynogens was verified using the isozyme locus ADA*. Analyses of sex ratios of meiotic gynogens suggested that male progenies were produced by an epistatic sex determining locus (SDL-2 with two alleles SR and sr) causing female to male sex reversal in the homozygous phase (srsr) but with limited penetrance. A close linkage was found between a sex determining locus (SDL-2) and the red gene. No significant difference was found between colour genotypes (namely homozygous red, heterozygous red and wild type) in terms of total fecundity, ISI (inter spawning interval), egg size and survival rate. Overall mean ISI was 26.3±1.0 days. Mean total fecundity was 1096 eggs. Fecundity varied over successive spawns but this variation did not appear to be related to spawning periodicity. Hormonal and thermal feminisation were compared on all YY male progeny of 0. niloticus. While similar female percentages of 32.0±5.2 and 33.8±1.5% were produced, significantly higher intersex percentages of 18.5±2.5 and 1.6±0.8 were observed in heat and DES treated groups, respectively. Heat treatment groups showed the lowest survival rate of 62.6±9.8% compared to the survival rates of 97.0±0.9% and 97.3±0.8% in controls and DES treated groups, respectively. YYRR males and YYRR neofemales were produced by integrating existing YYrr males and YYrr neofemales from the Egypt-Swansea-Philippine isolate and YYRR androgenetic males from the Stirling isolate with XXRR females and XYRR males of the Stirling isolate of Egyptian strain 0. niloticus. In summary, this study provides valuable information regarding the colour and sex determination mechanisms of 0. niloticus. The research in this thesis also demonstrated that both YY genotype and red coloration can be combined in a single strain in order to produce all male and stable red coloured 0. niloticus.