Beating cilia are important organelles for swimming in many zooplanktonic aquatic organisms, including many invertebrate larvae, rotifers and ciliates, but other planktonic organisms, such as copepods and brine shrimps, use muscle-powered swimming appendages. In recent studies we found that the temperature-dependent viscosity of seawater is the key physical/mechanical fac-tor that controls the beat frequency of water-pumping cilia in mussels and the swimming velocity in a ciliate. The present study on the swimming velocity of 3 zooplankton organisms, however, shows that the response of swimming velocity to a change in viscosity is different when due to a change in temperature or, at constant temperature, due to a manipulation of viscosity by addition of a high-mol-ecular-weight polymer (polyvinyl pyrrolidone, PVP) to the ambient seawater. There is a biological effect (fraction of total reduction of swimming velocity for a 10°C temperature reduction) that is found to be largest for the brine shrimp Artemia salina nauplius (37%) and the rotifer Brachionus plicatilis (26%), but negligible for the copepod Acartia tonsa (4%). We suggest that experimental data on change in swimming velocity (V) due to change in kinematic viscosity (ν) be correlated in terms of a power law, V ∝ ν-m. The present data on swimming velocity of copepods, brine shrimps and rotifers show values of exponent m ≈ 1.5 to 3, with a trend of decreasing values for increasing size of species. Differences in m-values may be ascribed to differences in propulsion system, body drag and size.