Three mutants of bacteriophage P1 affected in their ability to maintain the lysogenic state stably are described here. These mutants were normal in lytic growth, but lysogenic derivatives segregated nonlysogens at abnormally high rates (1 to 30% per division). Cells harboring these mutant prophages were elongated or filamentous. The mutations responsible for this prophage instability fell into two classes on the bases of their genetic location, their effect on the ability to lysogenize recA bacteria, and their suppressibility by ant mutations eliminating antirepressor activity. The two mutants that were able to form recA lysogens showed the same prophage instability and partial inhibition of cell division in recA as in rec+ lysogens. The fact that plasmid-linked mutations can cause prophage instability suggests that P1 codes for at least some of the functions determining its own autonomy and segregation.