The phage-typing modifications induced by transfer of antibiotic-resistance plasmids wre studied in two S. typhi Vi+ strains: n 2411 (phage-type A) and Ty2 (phage-type E1a). Forty-one R plasmids belonging to twenty-two incompatibility groups were investigated. Twenty-two plasmids were unable to produce any phage typing modifications. Among the groups of plasmids with phage-typing restriction capacity,four (I1, 10.B.O., N and W) groups were found heterogeneous with regard to this property and one (F1 group) caused significant modifications of the phage-types defined by the Vi phage-typing international system.