A seven-compartment model of the mixed layer ecosystem was used to fit a time series of observations derived from data obtained during the 1989 JGOFS North Atlantic Bloom Experiment. A nonlinear optimization technique was used to obtain the best fit to the combined observation set. It was discovered that a solution which gave a good fit to primary production gave a bad fit to zooplankton and vice versa. The solution which fitted primary production also showed good agreement with a number of other independent data sets, but overestimated bacterial production. Further development is necessary to create a model capable of reproducing all the important features of the nitrogen flows within the mixed layer.