The intrastructure of mycelial pellets of Streptomyces fradiae, which produces tylosin, was visualized following labeling with fluorescein isothiocyanate (FITC) and propidium iodide (PI) using confocal scanning laser microscopy. A PI-labeled inactive core was present inside the mycelial pellet in both fermenter and air-lift reactor cultures. The thickness of the active mycelial layer on the pellet surface was calculated from a density sliced image to be 60 and 68 mu m respectively, in the fermenter and in air-lift reactor cultures. Using image analysis, the active mycelial concentration of pellets in the fermenter culture was predicted to be 2.1 times higher than that in the air-lift reactor culture. The tylosin production rate in the fermenter reached 0.78 g/l/d, which was 2.5-fold that in the air-lift reactor culture. These results indicate that the higher tylosin production rate in the fermenter culture was due to the higher active mycelial concentration in the fermenter compared to that in the air-lift reactor.