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SUMMARY: The morphology of Mycobacterium lepraemurium, Hawaiian strain, cultivated on a glass slide in a cell-free liquid medium (NC5) at 30 °C for 12 weeks, was observed every 2 weeks with a scanning electron microscope. Flourishing growth and microcolonial growth occurred in the peripheral and central areas, respectively, of a smear. Elongation, septum formation, division, budding and branching in the bacteria were seen between the second and fourth weeks of incubation, which may correspond to the beginning of the exponential phase. After this period, the number of bacilli suddenly increased, and intertwined, elongated cells with large granules appeared. These features gradually increased until the stationary phase was reached after 12 weeks of incubation. These observations are the first scanning electron microscopical descriptions of the growth patterns of Mycobacterium lepraemurium in vitro.