Forty five amber mutants of Salmonella phage P22 were assigned to 12 complementation groups. Two amber mutants could not be classified into any one particular group. Ten of the complementation groups were mapped by two-factor crosses. The map is circular and about 65 units long. The remaining two complementation groups each consist of a single member which form very small plaques. Attempts to map these two mutants by two-factor crosses were unsuccessful.