We have observed from Uhuru large-amplitude X-ray pulsations from Cyg X-1, which occur several times a second with a duration of less than a fraction of a second. The amplitude of the pulsations at times exceeds 25 percent of the average source intensity. The pulsations do not occur at random. Although we cannot within our data uniquely determine their period, we find that the data are consistent with a period of 73 milliseconds. In addition to fine-scale time vanations, the average X-ray intensity from the source changes by factors of 2 over times of the order of 102 seconds. We conclude that we have discovered a pulsating X-ray star, whose characteristics are quite different from those of NP 0532.