Fifteen cointegrates of the plasmid Flac and prophage lambda that had suffered no detectable change in plasmid phenotype were isolated and characterized. The locations of the prophage insertions were determined by genetic analysis of deletion mutants obtained from each cointegrate as survivors of growth at 42 degrees C. In 11 cointegrates, the prophage was inserted between traI and lac, although probably in more than one location; in 3 others, it was on one side or the other of lac; and in 1 it was between lac and pif. Deletions covering all or part of the transfer region, as well as of lac and of pif, were obtained in the course of this analysis. Deletion mutants that had lost all known transfer genes were also oriT, but they retained the capacity to recircularize after transfer. Attempts were made to isolate lambda transducing phages for nearby plasmid genes from the cointegrates, and lambdaptraGD, lambdaptraD, lambdaptraI, and lambdadtraDI phages were obtained.