A manually operated pump for pressurized dispensing of a fluid product, designed to be fastened to the mouth of a container containing the product. The ensemble of the stem, spring, piston, and retention element form a unitary assembly that can be coupled to cup-shaped body of the pump. The pump includes also a member for connection of the unitary assembly to the cup-shaped body. The connection member includes a shaped portion for retention of the collar that projects from the stem, the portion preventing the exit of the stem from the cup-shaped body when the unitary assembly is inserted into the cup-shaped body.