Repeated transfers in 5 per cent peptone or asparagine media of sensitive B. megatherium cultures containing C phage have made possible the isolation of lysogenic bacteria which produce different strains of phage. This is probably due to the failure of C virus to make the lysogenic adjustment while those of its variants which can succeed are selected.

Four distinct plaque types have been isolated by this method and some of their reactions in mixedly infected bacteria are described.

It has been shown that it is possible to isolate either C or T analogues of each of the strains, bringing the reported plaque types of B. megatherium phage to eight.

No success was obtained in an attempt to isolate a wild type and a phage carrying two mutations when crosses were made of two phages each differing by one mutational step.

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