The DNA synthesized in the nonpermissive host by the noncomplementing mutants am36 and am42 of bacteriophage phi W-14 contains about half the wild-type level of alpha-putrescinylthymine (putThy) and a correspondingly greater level of thymine. The mechanisms whereby thymine nucleotides are excluded from replicating DNA are functional in both mutants because neither of them incorporates exogenous thymidine into DNA. It is proposed that (i) in wild-type phi W-14, the conversion of hydroxymethyluracil to putThy at the polynucleotide level is sequence specific, but that to thymine is nonspecific; and (ii) in the mutants, the sequence-specific recognition is impaired so that more thymine and less putThy are formed. The thymine-rich DNA can be packaged into phage particles. In the case of am42, the phage particles are morphologically indistinguishable from and have essentially the same polypeptide composition as wild-type particles. However, the DNA molecules they contain are about 11% shorter than those in wild-type phage, am42rev4, a revertant of am42, contains DNA with about 70% of the normal level of putThy; these molecules are about 3% shorter than wild-type DNA. The properties of am42 and am42rev4 are consistent with the suggestion that putThy facilitates the very tight packing of phi W-14 DNA (Scraba et al., Virology 124:152-160, 1983). It also appears that the putThy content of phi W-14 DNA can be reduced by no more than 30% without adversely affecting the production of viable progeny; for example, the burst size of am42rev4 is about 25% of that of the wild type.