The application of a pulsing flow to fluidized-bed bioreactors in order to control pellet morphology of filamentous fungi was investigated. The operation at an optimum pulsation frequency allowed two effects: a narrower pellet size distribution which improves fluidization quality, and an enhanced production of citric acid by Aspergillus niger and manganese peroxidase by Phanerochaete chrysosporium. In the case of A. niger, the pellet diameter corresponding to the pulsed system operated at 0.35 s-1 was kept between 3.3 +/- 0.1 mm after 22 days of operation; however, in the nonpulsed bioreactor which was operative for only 11 days, pellets with a diameter of 6.7 +/- 0.3 mm were produced. Similar results were obtained in the case of P. chrysosporium, since with a pulsing frequency of 0.0625 s-1, a pellet diameter of 2.1 +/- 0.4 mm after 34 days of operation was maintained. On the contrary, the system without pulsation presented great conglomerates of mycelia with an average diameter of 3 cm surrounded by free pellets with a diameter distribution of 2.75 +/- 0.5 mm. The nonpulsed bioreactor was only operative for 14 days. Both citric acid and manganese peroxidase production attained higher values and were maintained longer in the pulsed bioreactor.