In calculating the natural frequencies of a power chain or belt it is usual to use the theory of vibrating strings. However, the speed of the chain or belt may be a considerable fraction of the wave velocity as given by (T/ρ)1/2. It might be expected hat this would have a marked effect on the natural frequencies. This is shown to be so and the formula for the natural frequencies of a moving string is obtained. The problem of the forced vibration of the string is also considered.