An experiment was conducted in randomized complete block design with four replications in the experimental field of Botanical Garden, Department of Plant Breeding and Genetics, Sindh Agriculture University, Tandojam for assessing the heterosis, heterobeltiosis, inbreeding depression, combining ability, correlation and path coefficient analysis, genetic parameters and gene action for yield and yield components and seed viability in a set of 5x5 complete diallel cross of Gossypium hirsutum L. Phenotypic correlation coefficients (PCC) revealed that seed cotton yield/plant expressed strong positive (P≥0.01) association with sympodia/plant (r=0.863**), bolls/plant (r=0.921**) and boll weight (r=0.498*). Path coefficient analysis showed that the direct effect of bolls/plant and boll weight on seed cotton yield/plant was very high; 0.991 and 0.515, respectively and formed 118.94% and 183.99% of the total phenotypic correlation coefficient in F 2 generation. Heterosis was manifested in different crosses for all the characters but the extent of heterosis varied between characters. It may be assumed that heterosis in one or more than one yield components conferred heterosis for seed cotton yield. The cross Reshmi x TH-3/83, Reshmi x Mc-Niar-3150, CIM-109 x Mc-Niar-3150, CIM-109 x TH-3/83 and Mc-Niar-3150 x CIM-109 displayed positive heterosis and heterobeltiosis for seed cotton yield/plant alongwith one or more than one yield component characters. General reduction of heterosis in F 2 and in F 3 generation was due to decrease in heterozygosity through allelic fixation. Generally, predicted or expected inbreeding depression was quite higher than that observed for all the quantitative and qualitative traits except seed and lint indices, the discrepancy between the observed and expected inbreeding depression may be attributable to several reasons that involve linkage disequilibrium, epistasis and abnormal segregation at meiosis due to higher ploidy level of Gossypium hirsutum L. GCA variances were significant for plant height, sympodia/plant, bolls/plant, seed index, boll weight, lint index, G.O.T.% and seed cotton yield/plant, while SCA variances were significantly for plant height, bolls/plant and seed cotton yield/plant indicating additive and nonadditive type of gene action involved in the inheritance of these characters. Estimates of GCA effects of the parents indicated that TH-3/83, NIAB-78 and Reshmi were good generalxiii (xiii) combiners for seed cotton yield in F 2 generation due to their positive GCA effects. Parent Reshmi was also good combiner for plant height, boll weight, seed and lint indices, G.O.T.% and staple length followed by TH-3/83. Number of sympodial branches was of little importance in ranking the parents because parents that were good general combiner for yield and number of bolls did not hold a similar position for sympodial branches. The cross NIAB-78 x TH-3/83 performed better from rest of the crosses and displayed positive SCA effects in 10 out of 12 characters followed by cross NIAB-78 x Reshmi which displayed significant SCA effects in 9 out of 12 characters and express positive effects for yield and bolls/plant and reasonably positive effects for seed and lint indices. These crosses will form the superior combinations for hybrid crop improvement. Broad sense heritability computed through variance component method showed that all the quantitative traits were highly heritable in F 2 and F 3 generations except sympodia/plant. This suggests that selection for quantitative traits would be more rewarding than selection for qualitative traits in early segregating generations such as F 2 and F 3 . The trait seed cotton yield exhibited 73% broad sense heritability coupled with 31% expected genetic advance suggesting the existence of sufficient amount of genetic variability for improvement of this trait and also indicates that the trait is more amenable to selection and could be improved easily. The genetic components of variance for plant height, bolls/plant and boll weight in F 2 generation revealed that dominance components (H 1 and H 2 ) and additive component (D) were highly significant. It was further observed that dominance components were greater than additive components. Genetic components for seed cotton yield revealed that the dominance components were non-significant and greater than additive components. The average degree of dominance was more than unity, displaying over dominance type of gene action. The positive non-significant F-value revealed that the dominant genes were less frequent than recessive genes in the parents and that they were in decreasing position as exhibited by h 2 value. Estimated ratio of h 2 /H 2 indicated that there were atleast three groups of genes controlling seed cotton yield/plant. The value of r 2 showed the complete dominance of parents. Seed vigour was measured through the germination percentage and electrolyte leackage of seeds. Hybrids Reshmi x NIAB-78 and CIM-109 x TH- 3/83 displayed maximum germination percentage at 30 and 35 o C, of seeds from bottom portion of the plant and these crosses displayed moderate electrolyte leackage and were considered as vigourous seed when taken from bottom and middle portions of the plant.