A comparison of the rates of synthesis of the tryptophan biosynthetic enzymes of Salmonella typhimurium under derepression showed that the genes of the trp operon can be expressed in a coordinate fashion in auxotrophs carrying nonpolar mutations. This coordination disappeared in trpA polar mutants. The loss of coordination affected only trpB , the second gene in the operon, which was always more drastically affected than the three distal genes. Polar mutations in trpA , the first gene of the trp operon, reduced the rates of synthesis of the tryptophan biosynthetic enzymes under conditions of derepression. When these rates were measured and correlated with the map position of each polar mutation, a polarity gradient of decreasing intensity (moving distally from the operator end of the gene) was obtained. Certain mutations (“unusual mutations”) mapping at the operator distal end of trpA , and considered by other workers to correspond to the operator proximal end of trpB , were found to be polar. The bearing of our observations on the question of coordinate versus semicoordinate expression of the trp genes and the status of the “unusual mutations” is discussed.