a fluid mixing device which can adjust for temperature fluctuations of fluids wherein a thermal expansion element drives a reciprocating member and has a portion extending into and through a fluid inlet . in a preferred embodiment , the inlet is a common inlet for both hot and cold fluids and the flow of fluid is controlled as it exits from the mixing device . a fluid mixing device which is highly responsive to fluid temperatures results and is particularly suited for use in plumbing fixtures of various types .