The performance of a cyclone bioreactor for the cultivation of the filamentous microorganism, Aspergillus niger, was investigated. The research focused on the influence of the recirculation pump type (centrifugal and rotary piston) and speed, on biomass production and cell morphology (quantified using image analysis). In the absence of recirculation, the cell morphology was highly branched with several main hyphae. Increasing the speed of the centrifugal pump resulted in significantly shorter cells with a distinct main hypha and several short branches. Increasing the pump speed significantly increased the length of the lag phase of the cell growth. Results with the ’low shear’ rotary piston (Waukesha) pump yielded little or no biomass production and produced cells that were significantly shorter and thinner.