The morphology of mycobacteria is usually examined by taking a sample from colonies already developed on the medium surface. By this method, Mycobacterium avium complex is considered to form short rods or a little longer rods and occasionally some filaments. We inoculated test strains on Ogawa egg medium slants by a loop and incubated at 37 degrees C. After incubation for 2, 3 and 4 days, unvisible or only scarcely visible growth was taken by loop and placed on slides. The samples were stained by the Ziehl-Neelsen method and observed microscopically. Of 50 strains of M. avium complex examined, 4 strains showed mycelia after incubation for 3 days. These mycelia were not observed after incubation for 7 days when visible colonies developed but rods were observed. Other 46 strains did not show mycelium but showed filamentous growth after 3 days. The change observed was very similar to morphological change of nocardiae which first formed mycelia and thereafter the mycelia were divided into rods. In other slowly growing mycobacteria, M. tuberculosis, M. bovis, M. kansasii, M. scrofulaceum, M. gordonae, M. marinum and M. nonchromogenicum, such mycelium formation was not observed. The mycelium or filament formation in early stage of the growth seemed to be specific for M. avium complex together with M. xenopi.