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Selection for resistance to phage χ led to the isolation from Salmonella typhimurium strain LT2 of fifteen non-flagellate (fla -) mutants due to spontaneous mutation at sites such that H 1 could be co-transduced with fla +. Treatment with phage P22 grown on LT2 fla + evoked swarms, i.e. motile (fla +) transductant clones, from all fifteen fla - mutants and from a fla - strain of S. paratyphi B; and also evoked trails, i.e. fla + abortive transductants, from all these mutants except four. When the fla - strains were crossed with each other by transduction all pairs yielded fla + clones, which indicated that none of the mutated sites was identical with, or overlapped, any other. In most pairs the appearance of trails showed that abortive transductants, of constitution fla1-fla2+/fla1+fla2- , were flagellate, as a result of complementation. The mutants fell into five groups, such that mutants of any one group complemented mutants of all other groups; these groups perhaps correspond to five fla genes. Some pairs of the ten strains of group A complemented, perhaps through intragenic complementation.