After infection of Bacillus subtilis 3610 with bacteriophage ∅e, which contains hydroxymethyluracil in place of thymine in its DNA, bacterial DNA synthesis is arrested. Our results show that the phage-coded deoxythymidine triphosphate hydrolase (TTPase) is not essential for the arrest. A phage-coded protein made in the first 13 min after infection appears to be involved in arresting host DNA synthesis. Phage mutants defective in this function have been isolated. The inhibition of host DNA synthesis does not appear to take place by detachment of bacterial DNA from the cell membrane or by inhibition of DNA polymerase. A phage фe mutant defective in TTPase, with thymine replacing up to 20% of the HMU in its DNA, can grow and transfer the thymine-containing DNA to progeny normally. The role of HMU in this phage remains unexplained.