CO-modulation is a new type of modulation for the transmission of telegraphic signals. Its primary object is to increase the speed of transmission in a given bandwidth. The essential features of CO-modulation are: 1. The lower frequencies in the information signals are suppressed by means of a high-pass filter before modulation on a carrier. 2. Two channels are provided in the same frequency band by using two carrier-voltages differing90^oin phase. 3. The frequency components which are lacking are regenerated at the receiving end by means of positive feedback, thereby using a feedback network which is adapted to the high-pass filter mentioned in 1. The spectrum in the neighbourhood of the carrier frequency does not contain frequency components arising from the information signals. So a pilot signal of carrier frequency can easily be transmitted for phase-reference, thus allowing coherent detection of the signals transmitted by orthogonal modulation. In the demodulated signals the lower frequencies are missing. It is possible, however, to regenerate the low-frequency components suppressed at the sending end, because the related time-constants are well-known and independent of the transmission path. If these complementary signals are derived at the output and added to the received signals, then the original pulse pattern can be reconstructed. This procedure involves positive feedback which, for telegraphic signals, can easily be made stable. The name CO-modulation stands for complementary orthogonal modulation, thereby indicating the complementary action of suppressing and regenerating frequency components at the sending and receiving end respectively, and the orthogonal modulation by which two channels are supplied in the same frequency band. CO-modulation may be applied to synchronous as well as asynchronous telegraphy. In an experimental system the bandwidth from 700 c/s to 3100 c/s of a normal carrier-telephone channel was used for transmission of 4000 bauds. This corresp0nds to a relative speed of transmission of 1.67 bauds/Hz, which is not far from the theoretical maximum of 2 bauds/Hz as given by the sampling theorem.