When Escherichia coli cells lysogenic for bacteriophage lambda are induced with ultraviolet light, cells carrying cryptic lambda prophages are occasionally found among the apparently cured survivors. The lambda variant crypticogen (lambda crg) carries an insertion of the transposable element IS2, which increases the frequency of cryptic lysogens to about 50% of cured cells: 43 of these cryptic prophages have been characterized. They all contain substitutions that replace the early segment of the prophage genome (from the IS2 to near the cos site) with a duplicate copy of a large segment of the host chromosome. The right end of the substitution always results from recombination between the nin-QSR-cos region of the prophage and the homologous incomplete lambdoid prophage Qsr' at 12.5 minutes in the E. coli chromosome. The left end of the substitution is usually a crossover that recombines the IS2 element in the prophage with an E. coli IS2 at 8.5 minutes, near the lac gene, or with a second IS2 located counterclockwise from leu at 2 minutes, generating duplications of at least 200,000 bases. Five cryptic lysogens derived from cells lysogenic for a reference strain of lambda (which lacks the IS2 present in lambda crg) have been characterized. They contain substitutions whose right termini are generated by a crossover with the Qsr' prophage. The left termini of these substitutions are formed either by a crossover between the lambda exo gene and a short exo-homologous segment of Qsr' (2/5), or by a crossover between sequences to the left of attL and an unmapped distant region of the host chromosome (3/5). The large duplications carried by these cryptic lysogens are stable, unlike tandem duplications, and so may significantly influence the cell's evolutionary potential.