Enomoto, Masatoshi (National Institute of Genetics, Misima, Japan). Slow motile mutant in Salmonella typhimurium. J. Bacteriol. 90:1696-1702. 1965.-A slow motile mutant, SJ399, was isolated from a wild-type strain of Salmonella typhimurium TM2. The mutant was as motile as the wild type in broth culture at 37 C. However, on semisolid medium it produced a much narrower swarming band than TM2. The motility of this mutant was hindered by the viscosity of semisolid medium. H antigenicity and morphological characters of flagella of the mutant were the same as those of the wild type. The motility phage, chi, responded differently to SJ399 and the wild type. Plaques of SJ399 were small and cloudy, whereas on the wild type they were large and clear. The efficiency of plating on SJ399 was 0.36 as compared with 1 with the wild type. Stained preparations revealed that the mutant had about one-third the number of flagella of the wild type. The reduction of the number of flagella also was ascertained by biochemical measurement of flagellar protein which was purified after deflagellation from cells. The content of flagellin in SJ399 was about 32% of that of the wild type. Phage P22-mediated transductions from SJ399 to nonflagellated (fla(-)) and paralyzed (mot(-)) mutants showed that the mutant SJ399 complements seven fla(-) and three mot(-) strains which are representative mutants of flagellation and motility cistrons, respectively. The mutation site of SJ399 was cotransduced with both motA and B cistrons. The two point cross tests between SJ399 and mot mutants revealed that the mutation site of SJ399 is located in the motB cistron. The insertion of the genetic region containing the mutation site of SJ399 to the motB cistron is discussed in relation to intracistronic complementation.