The effects of cefotetan (CTT, YM09330) and cefmetazole (CMZ) on the morphological changes of Escherichia coli NY-17 and Serratia marcescens IID 620 were examined with scanning electron microscope. 1) CTT and caused E. Coli NY-17 filamentous forms at 0.024 microgram/ml but CMZ showed same morphological changes at 0.2 microgram/ml which is about 10 fold higher concentration than that of CTT. Spheroplast like structures and cell lysis were observed at lower concentrations than those of CMZ. 2) In case of Serratia marcescens IID 620, filamentous forms, spheroplast like structures and cell lysis were also observed at lower concentrations than those of CMZ. The action of CTT against the both strains was bactericidal.