Continuous fermentations were conducted with in situ cell retention using spin filters (pore size 5 mum and 10 mum) for propionic acid production by Propionibacterium acidipropionici. Continuous fermentation with a 5-mum pore size spin filter resulted in 50% cell retention. Propionic acid productivity was enhanced (0.9 g l(-1) h(-1)) by approximately four-fold compared to conventional batch fermentation (0.25 g l(-1) h(-1)). The in situ cell retention (5-mum pore size spin filter) bioreactor was operated continuously and smoothly for 8 days at a dilution rate of D=0.05 h(-1).