Mutants (N−nin) of bacteriophage λ in which the N gene product is not required for growth on wild-type Escherichia coli do not plate on recA bacterial mutants. Secondary mutants, selected for growth on recA, lie within the immunity region to the right of gene cI and appear identical to the cro mutants of Eisen et al. In an N+ phage, a cro mutation causes enhanced and prolonged production of λ exonuclease. N−cro phages make no detectable exonuclease, but show an increased rate of specific excision from lysogens and are excluded by P2 prophage. These properties, together with the ability to plate on recA, suggest that N−cro phages express genes to the left of N at a rate that is very low but higher than that for N−cro+ phages. N−nin phages can integrate at the normal site on the bacterial chromosome, but specific excision from lysogens is immeasurably low.