After analyzing histograms of Streptomyces virginiae classified by their oval major axes using image analysis, their morphology was differentiated into three types : free filaments, entangled filaments, and pellets. IL was found that the morphology of S. virginiae changes from free to entangled filaments after the depletion of limiting components, which can be detected by a sudden decrease in the percentage of CO2 in the exhaust gas, after which autoregulator synthesis is soon initiated. Adding virginiae butanolide-C (VB-C), one of the autoregulators synthesized in the cells, to the medium during the early growth phase did not induce the morphological change. It was also found that the morphological change had no direct relationship with virginiamycin production. That is, both the change in morphological shape and autoregulator synthesis arose from the depletion of limiting components, but once the autoregulator synthesis had started, the induced production activity was maintained without any clear correlation to the morphological shape. It was thus demonstrated that a simple image analysis parameter, in this case the oval major axis, could be utilized to detect the morphological change in S. virginiae. However, unlike some other cases of secondary metabolite production by filamentous microorganisms, the morphological change was not the key event for virginiamycin production.