A case of colistln nephrotoxicity without history of previous renal disease was studied by light and electron microscopy of a renal biopsy specimen obtained 11 days after discontinuation of the drug. The histologic picture showed variation from early acute necrosis to regeneration of the damaged renal epithelium. There appeared to be difference of susceptibility among individual tubular epithelial cells to sodium colistinmethanesulfonate in the development of nephrotoxicity. Morphological changes were essentially similar in both the proximal and distal tubules. Indentation of the nuclear membranes was conspicuous and appeared to be demonstrable earlier than the cytoplasmic alterations. ACTA PATH. JAP. 19: 55˜67, 1969