Approximately 6% of Escherichia coli K12 (lambda wild-type) cells whose prophage was induced by treatment with N-methyl-N-nitrosourea initiated plaques on E. coli K12S which contained wholly or mostly clear plaque-forming mutants (lambdac). "Fuzzy" plaque-forming mutants (lambdaf) were also recognised, at lesser frequencies. Less marked mutation occurred during prophage induction by N-ethyl-N-nitrosourea, and no apparent mutation occurred during induction by methyl and iso-propyl methanesulphonates, or by a non-inducing treatment of the lysogen with ethyl methanesulphonate. Mutagenic effects of treatment of susceptible host cells or of phage alone, prior to infection, seem not to account for the phenomenon described.