We present laboratory experiments designed to investigate the relationships between physiological traits (metabolic rate, survival time and weight loss under starvation), demographic parameters (maximum rate of increase, carrying capacity, population stability) and fitness measurements (ability to spread in either high- or low-density populations) in two clones of Daphnia magna STRAUS. Clone I had a higher maximal rate of increase and did better at low density; clone C had a more stable dynamics and a better competitive ability at high density. Carrying capacities were similar. No difference in metabolic rates could be detected; animals from clone C however survived longer under starvation, and died at a lower relative weight, thus suggesting a difference in the levels of stored energy. Age also had a significant effect, with younger animals surviving less under starvation. These differences in storage allocation are proposed to be responsible for the differences in population dynamics and fitness documented here.