the production of penicillin was carried out in a 100 m fermentor.phosphorus contents at various stages of 16 batches of normal production withour phosphorus feeding were investigated. In each batch, samples of every 10 h up to 60 h and every 24 h after 60 h were tested for phosphorus concentration in broth and mycelia. As shown in Fig. 1, at the first 30 h of logarithmic growth, the phosphorus content of dry mycelia was maintained at a level of approximately 0.0270 g/g because sufficient phosphorus source was present in the basic fermentation was medium. However, it started to decrease gradually after 30 h and reached to 0.0210 g/g at 60 h. Meanwhile, the withdraw procedure was carried out at 60 h and the removed biomass was replaced with growth media. As a result, The phosphorus content in dry mycelia decreased faster and 0.0114 g/g at the end of fermen- tation. The phosphorus concentration of the broth was 0.08 g/l at the beginning of the fermentation and was almost depleted at 30 h. It remained undetectable afterwards. These result indicated that the traditional procedure of penicillium production was likely not optimal because insufficient amount of phosphorus was supplied. It is obvious that phosphorus feed- ing during repeated fed-batch cultivations was necessary to maintain the phosphorus balance, especially during the pro- duction phase for the mycelium growth