As a result, almost the same increas-ing patterns were observed for each case, while the biofilm mass at the end of the culture was proportional to the medium depth. These results suggest that the thickness of the effective biofilm for cordycepin production is equal over a medium depth range from 1.8 to 5.3 cm, namely, the utilization efficiency of the biofilm decreases with an increase in the medium depth over this experimental range. The most effective point might be found at a lower medium depth than that of this experiment if further investigations are physically possible.