SUMMARY: The propagation of Vi-phage II on ‘degraded Vi strains’ of Salmonella typhi did not yield phage preparations bearing any trace of the specificity of the original Vi type from which the degraded strains had sprung. Ten different standard Vi-typing phages were grown on a selection of degraded Vi strains. The phages could be separated into two groups: six underwent a change which appeared to be a phenotypic modification, and four were propagated unchanged. It seems probable, however, that all adapted Vi-typing preparations are phenotypic modifications of Vi-phage II. In the attack of Vi-phage II on the typhoid bacillus the Vi antigen apparently plays a part only in the adsorption of the phage. The stages of phage growth succeeding adsorption seem to be independent of the Vi antigen, and it is in the regions of the bacterial cell concerned with these stages that Vi-type specificity seems to reside.