A general description is given of several effects of a temperature sensitive mutation in the T4 regulatory gene, mot. A reconstruction experiment illustrates an effect of the tsG1 mutation which can explain why this mutation was selected. The temperature dependent effects of the tsG1 mutation on DNA synthesis and phage production are described. An SDS-polyacrylamide gel analysis supports the notion that mot gene mutations cause extended synthesis of some early proteins whose synthesis is not extended after infection with DNA negative mutants. Data presented on the effect of the tsG1 mutation on the self-regulatory capacity of genes 43 and 32, the synthesis of mature T4 tRNA and the amount of translatable mRNA can be most easily interpreted as showing a transcriptional defect in mot gene mutant infected cells. Isolation of an amber mutation in the mot gene indicates that this gene codes for a protein. A temperature-shift experiment shows that inactivating the mot gene protein late after infection can have an effect on the expression of early genes.