In a tank for a system for injection supply of LPG for internal-combustion engines, a number of hydraulic-connection elements associated to a service flange which closes the top opening of the tank are mounted on said flange by fixing to the bottom surface of the latter, without having any parts that are mounted via through holes of the flange. Provided inside the tank is an electrically operated pump which is fixed to the bottom wall of the tank structure and which in turn carries the structure of a level-sensing device of a capacitive type.