By the use of two adapted phage preparations of Typing phage II the S. weltevreden phage types 4 and 5 could be classified into two sub-types each and phage types 9 and 10 into three sub-types each. the 1094 strains of S. weltevreden could be classified into a total of sixteen phage types including the sub-types. The host range mutants of Typing phage II were distinct from the parent strain. After adaptation to two insensitive strains, one of the new preparations, phi IIA lost its affinity to some strains which were lysed by the parent phage strain but gained lytic affinity for a few others that were originally insensitive. The second preparation phi IIB showed an increase in lytic range as expected. Antigenically these preparations were shown to be related but not identical. The possible reasons for serological non-identity of host range mutants with the parent strain have been discussed.