According to negative-staining electron microscopic observations, ethanol-sensitive Escherichia coil phages, ES1 and ES2, treated with 30% (v/v) ethanol, exhibited contraction of the tail sheath and released nucleic acids from the phage head shell. When these phages were treated with 50 or 90% (v/v) ethanol solutions, desorption of tail, contraction of the tail sheath and absence of DNA in the head shell were detected. Correlations were found between these phenomena and the survival rate of phages ES1 and ES2 treated with ethanol. In the ethanol-tolerant E. coil phages, ER1 and ER2, these phenomena were not observed by electron microscopy after treatment with 30% (v/v) ethanol and some of these phenomena to a small degree and no contraction of tail sheath could be observed after treatment with 70% (v/v) ethanol.