Repeated exposures of Shigella dysenteriae strain A to ultra-violetradiation (253.7 nm) with in-tervening outgrowth of survivors gave rise to clear bacteriophage plaques. Isolation, propagation and partial purification of the new Shd-4L10 phages showed that they are similar in morphology to the Myxobacteriaphage Mx-4described earlier. The new phages retained the general charac-teristics of S. dystenteriae phageShd-4L3,including serological properties and phage typing. It is suggested that ultra-violet irradiation may have played a role in the transformation and excision of the presumed lysogen of S. dysenteriaestrain A into a lytic phase. PhageShd-4L10was subse-quently partially characterized. It has a density of 1.61, a DNA: protein ratio 0f 0.42 and thus a cryptogram of D/2:54.3/32.5:X/X: B/O. The phage was further characterised by fractionation of its protein using SDS-polyacrilamide gel electrophoresis. DNA extracted from phages was hy-drolysed with restriction endonuclease R., EcoR1. The restriction fragments were catalogued and their apparent molecular weights calculated from electrophoresis gels calibrated with frag-ments from DNA of coliphage ë. From the total fragments obtained with nuclease R., EcoR1, the apparent minimum molecular weight of phage Shd-4L10DNA was found to be 54.3 x 106 Daltons. The molecular weight of the phage DNA was also calculated from measurements of contour length of purified DNA samples, using the formula MW = 1.97 x 1010 l/(magnification), where l is the measured length of DNA in centimetres). The very close relatedness with phage Shd-4L3 was confirmed by these techniques.