A small portable instrument based on the two-lamp sequence indicator is described; it is suitable for 3-phase supplies with voltages ranging from about 5 to 500 V, and frequencies from about 5 to 1500 c/s. It employs two rectifiers working back-to-back to produce a direct current whose polarity depends on the sequence of the supply voltages. Its experimental characteristics are given for sinusoidal supplies of variable voltage and of variable frequency, for unbalanced voltages, and for supplies containing harmonics.