Effects of ofloxacin on morphology of Mycobacterium fortuitum ATCC 6841 were studied electronmicroscopically. The cells treated with 0.2 microgram/ml (1/2 MICs) of ofloxacin at 37 degrees C for 24 h maintained normal morphology under scanning electron microscopic observations. In the ultrathin sections of the cells treated with ofloxacin under the same conditions, ribosomal granules and other cytoplasmic organellae were clearly observed, whereas the fine fibrous structures of the nuclear apparatus were not clear. On the other hand, the treatment with 0.2 microgram/ml of ofloxacin at 37 degrees C for 5 days caused elongation of the cells, and irregular, rosary-like and/or twisted features in some cells under scanning electron microscopic observations. The formation of numerous vacuole-like structures in the cytoplasm, the indistinct nuclear apparatus, and the partial destruction of the cell wall were observed in the transmission electron microscopic examinations. We conclude from this study that morphological changes in Mycobacterium fortuitum cells treated with ofloxacin seem to firstly occur in the nuclear apparatus.