A downhole electrical power generator. A downhole electrical power generating system includes a flow restricting device which variably restricts flow through an opening, the restricting device vibrating in response to flow through the opening and the restricting device thereby alternately increasing and decreasing flow through the opening; and an electricity generating device which generates electricity in response to vibration of the restricting device. Another downhole electrical power generating system includes a flow restricting device which vibrates in response to flow through an opening, the restricting device thereby alternately increasing and decreasing flow through the opening, a pressure differential across the restricting device variably biasing the restricting device to increasingly restrict flow through the opening, and the pressure differential alternately increasing and decreasing in response to respective alternate increasing and decreasing flow through the opening; and an electricity generating device which generates electricity in response to vibration of the restricting device.