Actively motile, flagellated phytoplankton taxa often exploit vertical gradients in the availability of light and nutrients. The ability to move actively usually goes along with low investment in structural carbon components and should entail costs in terms of increased energy expenditure. This should be reflected in higher metabolic rates and higher light requirements for growth and, consequently, in lower light-dependent specific production rates, carrying capacities, and carbon-to-phosphorus (C : P) ratios (during phosphorus limitation) of flagellated compared to nonmotile taxa. Laboratory experiments with four flagellated and five nonmotile species of green algae, performed over a light gradient, corroborated these expectations. Parameter fits to short-term production-irradiance measurements suggest that flagellated taxa had higher respiration rates and higher light requirements for growth than nonmotile taxa. Accordingly, both short-term photosynthetic rates and longer-term (14 d) biomass accrual were lower for flagellated than for nonmotile taxa. While most of the variance in algal C : P ratios was explained by species-specific effects, there was also a tendency for algal C : P ratios to be lower in flagellated that in nonmotile taxa. Collectively, these results point at significant costs of motility, which may explain why flagellated taxa are often outcompeted by nonmotile taxa in turbulently mixed environments, where active motility is of little use.