The effect of biotin-concentration on the growth and morphological character ofMicrococcus glutamicuswas studied, using seven strains of this organism. 1) Effect on the growth. M. glutamicus requires biotin for its growth in the synthetic medium. However, some differences of the biotin requirement were observed among seven strains used. Generally, at lower level of biotin-concentration (less than 1r/l), the cell growth of these strains was suppressed. On the other hand, at higher level of biotin-concentration (10 r/l or more), good growth of cells was obtained. 2) The effect of biotin-concentration on the formation of septa. M. glutamicususually formed elongated multicellular rod when it grew in the synthetic medium of biotin defficiency. In such case, many septa were found in the rod. Under the same level of biotin-concentration (biotin 3r/l or less), the cells which grew on the slanted agar-medium formed more numerous septa than those in the synthetic liquid-medium. The formation of septa was not observed in most of the cases when those strains grew in the synthetic medium containing 10r/l or more biotin.