. The present study has two parts. In the first (Chapters 2 an 3) Fisher's theory of sex ratio is extended and in the second (described in Chapters 4, 5 and 6) the methods of quantitative genetics were applied to a laboratory population of Drosophila melanogaster to evaluate the amount of genetic variance in sex ratio. In Chapter 2 the relationship between birth control, sex preference and sex ratio was considered. It has been suggested that in some human populations the desire for male or female offspring coupled with the knowledge of methods of birth control can influence the distribution of family sizes. The consequences of this situation have been analysed using the evolutionarily stable strategy concept. . In Chapter 3, using precise genetic models, I have studied the evolution of the sex ratio in a population in which interactions between siblings exist but these interactions are not symmetrical with respect to the sexes. Some interesting differences between cooperative and aggressive models have emerged. The second part of the thesis is composed of three experiments. Experiment I was designed for investigation of the variability in the sex ratio character and identification. of the sources of heterogeneity apart from that due to binomial sampling. The results showed a close agreement with the expectations from random segregation and indicated that genetic or environmental factors affecting sex ratio were virtually absent. It has been reported that environmental factors can "distort the sex ratio in several species. One such factor is the age of parents. Experiment II was performed to study this suggestion in Drosophila melanogaster in the hope of obtaining some information about the mechanisms involved in this distortion. The data indicated that this phenomenon, at least in D, rosophila melanogaster is non-existent. In Experiment III artificial selection for sex ratio was performed. Nine generations of selection were unable to increase the sex ratio of the population. Selection was very effective in increasing the number of females, but the presence of sex-linked lethals was shown to be responsible. It was concluded that all of the empirical evidence supports a Mendelian interpretation and gives no support" to any theory invoking" adaptive sex ratio.