Daily (circadian) rhythms exist at molecular, physiological, and behavioral levels and coordinate many life functions. This coordination is believed to contribute to an organism's fitness, however, such contributions have not been convincingly demonstrated in any animal. The most significant measure of fitness is the reproductive output of the individual and species. In this thesis I examine the consequences of loss of clock function on reproductive fitness in Drosophila melanogaster. I demonstrated that single mating among couples with mutated period (per���), timeless (tim���), cycle (cyc���), and Clock (Clk[superscript Jrk]) genes resulted in approximately 40% fewer progeny compared to wild-type flies. Male and female contribution to this phenotype was demonstrated by a decrease in reproductive capacity among per��� and tim��� flies mated with wild-type flies of the opposite sex. The important role of clock genes for reproductive fitness was confirmed by reversal of the low fertility phenotype in flies with rescued per or tim function. These results prompted an investigation to determine the relative contribution of each sex to the fertility phenotype. Males lacking a functional clock showed a significant decline in the quantity of sperm released from the testes to seminal vesicles (SV), suggesting that this peripheral oscillator is involved in sperm maturation. We found that clock genes are rhythmically expressed in these tissues and the cycling of per and tim expression continued in vitro, hence the testes and SV complex contained an autonomous circadian clock. In contrast to males, PER and TIM were constantly present in the cytoplasm of follicular cells in fly ovaries. Ovarian expression of per and tim is not disrupted by constant light and females lacking per and tim produced nearly 50% fewer mature oocytes then wild-type flies. These results suggest that per and tim are acting in a non-circadian pathway in the ovaries. Taken together, this data demonstrates that circadian clock genes significantly contribute to the fitness of Drosophila melanogaster by affecting the fecundity of both sexes. / Graduation date: 2003