An efficient method of growing the protozoon Tetrahymena to high cell densities in a 2-1 bioreactor is described. The first phase of the fermentation is a batch phase with minimum generation times (1.4 h). During the next phase medium is exchanged continuously using a perfusion module based on microporous hollow fibres while cell are retained. Compared to standard batch fermentations of this organism 30- to 40-fold higher cell concentrations and dry weights were achieved routinely. A maximum cell concentration of 2.2 x 10(7) cells/ml and a dry weight of 54 g/l have been obtained. As estimated from isocitrate dehydrogenase activity in the culture medium, no cell damage occurred even at high agitation rates. In addition, the cells remained viable for several weeks. Temporal limitation of the process was due to a decrease in the perfusion rate caused by blocking of the membranes. By X-ray microprobe analysis calcium phosphate depositions were detected in the pores of the clogged hollow-fibre membranes. However, even a T. pyriformis strain possessing mucocysts, dense core secretory organelles that may lead to early membrane clogging, was cultivated successfully for 3 weeks. Additionally, the consumption of nutrient protein and carbohydrates during fermentation was investigated and the effect of different perfusion rates and of glucose was studied in order to increase the efficacy of the system.