Size appears to be a very good indicator of physiological behaviour in the zooplankton community, like predation or even detritus production. In order to get a better understanding of planktonic ecosystem dynamics, we build a continuous sizestructured model. This approach is very popular in fish studies (Benoit and Rochet,2004; Andersen and Pedersen, 2009). Our model incorporates predation on phytoplankton and cannibalism within the zooplankton community, and various predation behaviours are tested. Mathematical results are in the most cases hard to get in such models. Nevertheless for some choice of predation function, and external mortality, we can obtain mathematically equilibria. Numerical simulations show that these equilibria are not always stable, and Hopf bifurcations appear when model parameters vary.