Morphological change during culture of the tylosin producer Streptomyces fradiae under various shear conditions was investigated using an image analysis system. The morphology was classified as filament, entangled filament, or pellet by measuring the mycelial area and convex perimeter. At fermenter agitation rates of 400 rpm and higher, filament and entangled filament morphology comprised more than 50%. How-ever, in cultures at agitation rates lower than 400 rpm, pellet morphology increased gradually and until it comprised more than 50%; in an air-lift reactor, the pellet ratio reached 80% after 160-h culture. The average areas of pellets and mycelia in the air-lift reactor were respectively 100- and 20-fold those under high shear condition. Both pellet area and mycelial morphology changed during the culture period according to the shear conditions. Among the various sheer conditions, tylosin production and rapeseed oil consumption were lowest in the air-lift fermenter culture with against in the fermenter at 400 rpm. From the viewpoint of tylosin production, the optimal mycelial area that gave the maximum production rate under various shear conditions was around 4 x 10(3) mu m(2). When the mycelia and pellet areas exceeded 4 x 10(3) mu m(2), the tylosin production rate decreased drastically.