An aquarium heater including a housing and a tube depending therefrom. An electric heating element is disposed in the bottom portion of the tube and a thermostatic control chassis is disposed in the upper portion of the tube. The control chassis includes two opposing housing sections matingly engaged in a nesting relationship to form a cylindrical housing unit which snugly fits into the tube and supports a bimetallic thermostatic switch together with associated circuitry and lead wires such that the thermostatic circuit elements are maintained securely positioned within the tube. The housing includes a gasket situated between a lip of the tube and the bottom wall of the housing, and a locking ring which clamps the rim of the tube in place. The tube is thereby held securely to the housing independently of the control chassis. A vertically slidable actuator arm engages a circular cam surface in the housing, where the cam surface converts rotary motion of a knob into vertical motion to thereby move the actuating arm. The lower end of the actuating arm engages and positions an adjustable contact member of the thermostatic switch to thereby maintain the temperature level of the heater as close as possible to a desired value.