This paper deals with the modelling of continuous bioreactors by neural networks. The detailed procedure of neural network modelling is given, along with comments on neural network topology, the training data set, and some key design parameters. Various simulations are carried out to study the feasibility of neural network modelling for this application. Simulation results obtained are satisfactory, concerning the modelling ability and noise rejection features of the neural network. It shows that the back-propagation neural network is a promising tool for bioreactor modelling.