Wild type phage HK022 was mutagenized by N-methyl-N'-nitro-N-nitrosoguanidine to induce clear plaque mutants. A total of 225 clear plaque mutants were isolated and 198 of these were assignable to one or the other of two complementation groups of the corresponding cistrons which have been designated as cI and cII, respectively. Approximately 25% of the c mutants were found to be temperature-sensitive (cts); producing turbid plaques at 32 C and clear plaques at 38 C and above. From complementation tests involving cI and cII mutants, bacteria lysogenic for cII prophage were frequently obtained. Double lysogens harboring a CI and a cII prophage were infrequently found and single lysogens harboring only a cI prophage have not been recovered. Bacterial lysogens harboring a prophage carrying a cts mutation in the cI cistron were readily obtainable. However, such lysogens show a lethal phenotype at 40 C and above, although they appear to be fully viable at 32 C. It is shown that by incubation of lysogens harboring a cts mutant of the cI cistron at 42 C, it is possible to isolate cryptic lysogens which are non-immune but harbor at least one of the phage sus+ alleles. Genetic data involving cI, cII, and two complementing sus mutants of essential genes are presented. From these data the following vegetative map is deduced: sus4--cII-cI-sus3.