Nonsense mutants of bacteriophage phi6 have been isolated and characterized. Eight classes have been identified involving 9 of the 12 proteins whose synthesis is directed by the virus. Pleiotropic effects have been observed among the mutant types. Mutants of class 1 and class 2 which affect proteins Pi and P2, respectively, direct the synthesis of only early proteins in nonsuppressor cells, indicating that these two proteins are involved in the transition from early to late viral protein synthesis. Polar effects are observed in several mutant classes. Virus containing a nonsense mutation in the gene for P8 does not direct the synthesis of P12 in nonsuppressor cells, whereas several mutants defective in the synthesis of P12 do cause the synthesis of P8. Similarly, cells infected with mutants that are defective in P9 also lack P5 and P11, while those infected with virus defective in P5 and P11 synthesize normal amounts of P9. A situation of reciprocal polarity exists between P3 and P6 in which the lack of production of either protein results in the reduced synthesis of the other. Evidence is presented indicating that the pair of proteins P5 and P11 are derived from a common precursor and that P5 is a cleavage product of P11.