This thesis presents a synthesis of experimental studies with Daphnia, in a multigenerational perspective. The main goal of the thesis was to investigate the role of diet (phosphorous (P) limitation) and temperature (10 ˚C, 20 ˚C) on various fitness-related traits, as well as diploid DNA content (2C-value). For a wide range of ectotherms, it has been demonstrated that individuals reared under reduced temperatures obtain larger body size. Daphnia may have a mixed strategy for this, i.e. they may increase their body size both by increasing cell size and cell numbers. It is further hypothesized that temperature and phosphorus limitation can drive changes in genome size, which in many cases scale with cell size and body size.To explore these ideas, 2C-value was estimated in two Daphnia species using flow cytometry. Significant genome size differences were found between the two species. D. pulex has a smaller genome compared to D. magna, and the estimates align well with previous studies. Additionally, differences were found in 2C-value between the two temperature regimes. However, this came along with increased coefficients of variance (CV), which is further discussed in the thesis. This study did not find evidence for the hypothesis that diet in respect to C:P ratio can induce changes in 2C-value over a modest number of generations.The growth rate hypothesis predicts positive relationships among growth rate body RNA and body P (% of dry mass). This study tested this within- and across-species by investigating the experimental lines reared in 20 ˚C under a “common garden” experiment with reciprocal transplantations. Additionally, the level of the enzyme alkaline phosphatase (AP) was measured in the daphnids, since it is hypothesized to be involved in P-metabolism in Daphnia. The present study find strong support for the growth rate hypothesis, and could reveal maternal and line effects for D. magna in some of the measured traits. Further, this study finds evidence for a significant decrease in AP activity in P-stressed daphnids.