Membrane reactors with a Proteus sp. strain as the biocatalyst were used to attain continuous L-carnitine production from crotonobetaine. Hollow fiber cartridges with fibers of polysulfone provided adequate filtration rates and were reusable after long-term operation (consecutive runs of 140 h followed). Running under optimum medium composition, these systems enabled high cell densities (35-45 g dry weight/1) to be reached after 35-40 h at dilution rates ranging from 0. 1 to 1.5 h(-1), while cross-flow filtration units with flat-sheet membranes made of polyvinylidene difluoride (PVDF) showed lower cell concentrations (up to 25 g dry weight/1) and levels Of L-carnitine production. The composition of the hollow fiber membranes probably improved filtration flow rates and definitely speed up the cleaning-up procedure because cake layers did not adhere tightly to the membranes. Furthermore, resting cell-cycles were maintained in continuous operation for 2-3 days with crotonobetaine and phosphate buffer as the only medium components, followed by cycles of cell re-activation, meaning that the system was stable for long periods of time (140-200 h). During resting cell-cycles, high cell decay rates were observed due to nutrient shortages. The hollow fiber system was able to produce high levels Of L-carnitine during the growing/resting cycles with 35-50% conversion and productivities as high as 40.5 g/(1h). The optimum dilution rate was 1.0 h(-1) as mass-transfer problems appeared at higher dilution rates because of high cell densities. The process described is presented as a way of recycling D-carnitine from the chemical industry, since, when dehydrated, this compound renders crotonobetaine.