The structural change of a filter cake and its effect on permeation flux in crossflow filtration of a suspension containing particles different in size were studied. A suspension containing yeast cells (5 mum in average diameter) and polystyrene latex particles (0.5 mum) with different compositions was used as a model. The change in the structure of the filter cake formed on the membrane surface was observed by using a scanning electron microscope. When the pore size was smaller than the latex particle size, a yeast cell layer containing latex particles was formed on the membrane at the initial stage of filtration. Then, the yeast cell layer was replaced by the latex particle layer. The increase of the circulation flow rate decreased the amount of the filter cake to increase the permeation flux in this case. On the other hand, when the membrane pore size was larger than the latex particle size, a yeast cell layer was first formed on the membrane surface followed by deposition of latex particles on it. The permeation flux was relatively higher than the former case. However, the permeation flux slightly increased with the increase of the circulation flow rate although the weight of the filter cake decreased in the latter case. Thus, a small amount of the latex particles in the cell suspension considerably decreased the permeation flux in crossflow filtration of the suspension. In addition the dependence of the permeation flux on the circulation flow rate was proved to be different whether the small particles were smaller than the membrane pores. These findings including the structure of the filter cakes will be helpful to understand the permeation behaviors in the crossflow filtration of microbial broths which usually contain small particles.