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the term fed-batch culture to describe batch cultures which are fed continuously, or sequentially, with medium, without the removal of culture fluid. A fed-batch culture is established initially in batch mode and is then fed according to one of the following feed strategies: 1. The same medium used to establish the batch culture is added, resulting in an increase in volume. 2. A solution of the limiting substrate at the same concentration as that in the initial medium is added, resulting in an increase in volume. 3. A concentrated solution of the limiting substrate is added at a rate less than in (1) and (2), resulting in an increase in volume. 4. A very concentrated solution of the limiting substrate is added at a rate less than in (1), (2), and (3), resulting in an insignificant increase in volume. Fed-batch systems employing strategies (1) and (2) are described as variable vol- ume, whereas a system employing strategy (4) is described as fixed volume. The use of strategy (3) gives a culture intermediate between the two extremes of variable and fixed volume