The optimal design of isothermal fixed-bed catalytic reactors for maximum outlet yield is studied by considering optimal feed distribution (OFD) along the reactor axis for parallel reactions. The activity distribution in pellets is assumed to have the Dirac delta type and the active location is the same for all pellets. The OFD function is derived analytically, indicating a uniform distribution along the reactor portion starting at the zero of the axial coordinate. The size of the feed distribution portion and the active location in pellets need to be found numerically, but in some particular cases, they can be found also analytically. A comparison of the OFD to the optimal catalyst activity distribution (OCAD), along the reactor axis shows that the OFD can give the same outlet yield as the OCAD does, while the former needs only one type of the Dirac delta catalyst activity distribution for all pellets in the reactor.