This thesis investigates patterns of genome size diversity in fishes. Genome size is known to be positively related to ecological amplitude and a range of characteristics along the 'r-K' continuum in many plant and animal groups. Positive associations between genome, nucleus and cell size, as well as negative associations with metabolic and development rates are known from all vertebrate classes but fishes, where the adaptive significance of DNA content is unknown. This study establishes that genome size is positively associated with cell and nucleus size in all fishes, which is consistent with an ' adaptive' interpretation of DNA content variation for this group, and supports the nucleotypic theory. Genome size is not associated with metabolic rate, but is positively associated with egg size and parental care, suggesting that fish genome size has been more strongly constrained by reproductive than metabolic parameters, as in other poikilotherms. Freshwater fishes have larger genomes than marine fishes, supporting a positive association between DNA content and ecological amplitude.