SummaryPhages were detected in 23 out of 56 cultures ofPs. mors‐prunorum, the organism causing the bacterial canker of cherry and plum in south‐east England, but not in 9 cultures ofPs. syringaefrom diseased pear trees. Attempts to free cultures from phage by repeated subculture from single colonies were in most cases unsuccessful, i.e. the majority of them were lysogenic and not carrier strains. This was confirmed for one strain by plating it with a suitable indicator strain, when a ratio of colonies to plaques slightly greater than 1:1 was observed. Lysogeny was encountered more frequently among the cherry than among the plum strains ofPs. mors‐prunorum. In typing experiments with 13 phages isolated from lysogenic cultures, the nonlysogenic cherry strains reacted more frequently than the lysogenic strains. No such relationship was observed among the plum strains, many of the nonlysogenic forms being immune to all the phages. Phage resistance in the cherry strains could be attributed to the presence of an homologous or closely related prophage, but it apparently had a different basis in the plum strains. The results confirmed the dissimilarity in the phage relationships between plum and cherry strains, previously observed with mainly virulent phages isolated from
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