The mixing unit for a mixer tap has a generally cylindrical shape defining a main axis and comprising a first intake of a first incoming stream of fluid having a first temperature, a second intake of a second incoming stream of fluid having a second temperature greater than the first temperature, an outlet for an outgoing stream, means for mixing the first and second incoming streams to form the outgoing stream, and thermostatic means which comprise a portion that is mobile in translation along a shutter axis. According to the invention, a plane that is orthogonal to the main axis and the shutter axis intersect, wherein the shutter axis is inclined with respect to this orthogonal plane, and passes through it so that the mixing unit, while being easy to manufacture, has sufficient compactness to accommodate most existing mixer taps.