A new, simple, rapid, and sensitive spectrophotometric method has been developed for the determination of manganese in sewage. The method is based on the reaction of manganese with derivatives of 1,3,3-trimethyl-2-[3-(1,3,3-trimethyl-1,3- H-indol-2-ylidene)propenyl]-3 H-indolium to form a colored ion associate with a sensitive absorption maximum at 560 nm. The appropriate reaction conditions have been established: pH 8.5-10.0, 1.25-2.3 x 10(-3) mol L(-1) 1-nitroso-2-naphthol, and 1.6-2.4 x 10(-4) mol L(-1) dye reagent. Beer's law is obeyed for manganese concentrations up to 4.2 mg L(-1). The limit of detection is 0.01 mg L(-1) Mn(2+); the molar absorptivity of the ion associate was 7.5 x 10(4) L mol(-1) cm(-1). The effect of various foreign ions was examined. A reaction mechanism is suggested. The developed procedure was tested for determination of manganese in sewage with satisfactory precision and accuracy.