The stability of FAD in mycelia of Eremothecium ashbyii was found to be as follows. (1) When mycelia containing FAD were incubated at 37℃ for an hour in the phosphate buffer of pH 5.3 and 7.4,their FAD content changed from 1.36mg per g to 1.26 and 1.27mg per g respectively. (2) When wet mycelia containing FAD were dehydrated with six volumes of dry acetone, the FAD content was decreased from 6.29 to 4.17mg per g. After this sample had been stored in a desiccator at room temperature for six months, little change in the FAD content was observed. (3) When mycelia containing FAD were made vermiform and dried at 75-78℃ for four hours, or when they were dried up on a hot porous the for 1-2 minutes, the FAD content was decreased from 6.26mg per g to 4.45 and 4.51mg per g respectively. These samples were stored similarly for six months and similar results were obtained.