The thermostatic cartridge (C) comprises a thermostatic assembly (2) for regulating the temperature of an outgoing mixture (F3) of incoming hot and cold fluids (F1 and F2), two ceramic disks (3, 4) for controlling the flow rates of the hot and cold fluids delivered to the thermostatic assembly, and a disk-actuating sleeve (5) receiving therein in leaktight manner the outer body (2.1) of the thermostatic assembly. To limit costs and any risk of leakage from the cartridge, the actuator sleeve is a single part that defines two internal channels (52, 53) for passing flows respectively of the hot and cold fluids from the disks to respective admission slots (2.2, 2.3) defined by the outer body of the thermostatic assembly.