A double-acting, simplex, fluid handling pump is designed to facilitate an optional configuration as either a plunger pump or a diaphragm pump. An injection molded plastic pump body comprises two bilaterally symmetrical halves that include internal pockets and grooves for clamping and retaining first and second generally identical valve assemblies at opposing end portions of the pump body. When configured as a plunger pump, first and second plunger are arranged to move 180.degree. out-of-phase with respect to one another within stationary guide sleeves that are clamped within the pump body where one plunger effecting a suction stroke while the other effects a compression stroke. When configured as a diaphragm pump, the plungers are removed from the connecting rods and replaced by diaphragms and a change is made in the valve casing employed but a majority of the remaining parts of the pump assembly remain unchanged from what is used in the plunger pump.