In order to better understand the effect of transient filtration parameters on the filtration performance during cross flow microfiltration of yeast suspensions, a laser distance sensor was used to measure on-line the height of the filter cake in a specially designed flat channel module. The data obtained with an accuracy of +2 mu m were used to fit a model describing the build-up of the filter cake and the related flux decline. The model is based on a volume balance over the deposited particles, with the flux of the particles back into the concentrate stream proportional to the average cross flow velocity at the centre of a particle placed on the surface of the filter cake. An empirical back transport coefficient was introduced to fit the data to the model. The results showed that with yeast suspensions membrane fouling is limiting the filtration performance. Also the performance cannot be improved by transient pressure conditions generated by an alternation of the flow direction in the module.