Abstract Ninety-six conditional lethal mutants of coliphage M13 have been isolated and assigned to complementation groups as part of a program to determine the number of cistrons and their functions in this small DNA virus. Genetic complementation tests show that the 63 temperature sensitive (ts) mutants isolated fall into 5 cistrons and the 33 amber (am) mutants into these same 5 and 1 additional cistron. With the limited evidence on hand it is not possible to conclude whether these comprise all the cistrons in the phage. Temperature shift experiments carried out on ts mutants from cistrons 1 through 5 show that cistron 2 starts to act during the early minutes of M13 infection, while cistrons 1, 3, 4, and 5 start to act only late in the latent period. The functions of all 5 of these cistrons are needed to maintain phage production once it has begun. The phage production stops immediately if any of the late functions are blocked, but only gradually after the early function is blocked. Mutants from the early and late cistrons differ dramatically from each other in their effect on host cells under nonpermissive conditions. Neither am nor ts mutants in the early cistron affect the viability or growth rate of the infected cells, while the mutants in all the late cistrons kill the cells, stopping their growth soon after infection. The wild-type M13 phage does not kill the cells under normal conditions of infection, but does decrease their growth rate. It appears that the host cells are killed when the early cistron is operative and phage production is prevented by a block in any one of the late cistrons.