A GaAs MESFET X-band oscillator has been frequency modulated by an amplitude modulated optical signal injected on the active device. A modulation index of 0.5 corresponding to MSK modulation has been achieved at 2 Mbit/s without parasitic amplitude modulation. It is shown that the FM modulation index variation against frequency is directly related to the drain current response of the FET considered as a photodetector. This responsivity has been measured to decrease above 5 MHz, which limits the oscillator FM capability.