The low tolerance of cyanophage AN-23 to NaCl at more than 0·05 M has been investigated. When isolated from clarified lysates that had been exposed to NaCl in the presence of added DNAse I, the phage genomic DNA consisted of a series of discrete fragments. The fragments were recovered from low melting temperature agarose electrophoresis gels and used for restriction analysis. Comparison of the restriction cleavage products with a restriction map of the 46 kbp AN-23 genome has shown that the fragments are generated by removal of DNA from one end of the linear genome in steps of about 3 kbp. The data are consistent with a model in which NaCl triggers the ejection of the phage genome from the capsid.