Mixing unit for a mixer tap, the mixing unit comprising: - a first inlet for a first incoming stream of fluid having a first temperature (Tf), - a second inlet for a second incoming stream of fluid having a second temperature (Tc) which is greater than the first temperature, - mixing means for mixing the first incoming stream with the second incoming stream to form an outgoing stream of fluid having an outlet temperature (Tm), - an outlet for the outgoing stream, - a closure member (72; 272) which is movable in translation, along a closure axis (X70; X270), between an open position of the second inlet and a closed position of the second inlet, - a housing conduit (70; 270) which connects the outlet to the second inlet while extending along the closure axis (X70; X270), - an actuating element (106; 306) for the closure member which is arranged at least partially at the outlet so as to deform, as a function of the outlet temperature, between: • a first shape for moving the closure member into the open position, and • a second shape for moving the closure member into the closed position, the mixing unit being characterised in that: - the actuating element (106; 306) is made of a shape-memory alloy; - the closure member (72; 272) comprises a pusher (74; 274) which extends in the housing conduit, by way of which the actuating element (106; 306) actuates the closure member, the pusher being formed by a rod which is mounted to slide along the closure axis in the housing conduit; and - the pusher closes the housing conduit (70; 270), the mixing unit comprising a sliding fitting (76; 276; 676) for sealing the closing of the housing conduit by the pusher. - a first inlet for a first incoming stream of fluid having a first temperature (Tf), - a second inlet for a second incoming stream of fluid having a second temperature (Tc) which is greater than the first temperature, - mixing means for mixing the first incoming stream with the second incoming stream to form an outgoing stream of fluid having an outlet temperature (Tm), - an outlet for the outgoing stream, - a closure member (72; 272) which is movable in translation, along a closure axis (X70; X270), between an open position of the second inlet and a closed position of the second inlet, - a housing conduit (70; 270) which connects the outlet to the second inlet while extending along the closure axis (X70; X270), - an actuating element (106; 306) for the closure member which is arranged at least partially at the outlet so as to deform, as a function of the outlet temperature, between: • a first shape for moving the closure member into the open position, and • a second shape for moving the closure member into the closed position, • a first shape for moving the closure member into the open position, and • a second shape for moving the closure member into the closed position, - the actuating element (106; 306) is made of a shape-memory alloy; - the closure member (72; 272) comprises a pusher (74; 274) which extends in the housing conduit, by way of which the actuating element (106; 306) actuates the closure member, the pusher being formed by a rod which is mounted to slide along the closure axis in the housing conduit; and - the pusher closes the housing conduit (70; 270), the mixing unit comprising a sliding fitting (76; 276; 676) for sealing the closing of the housing conduit by the pusher.