A precise measurement of the cross section for the procluctio~l of C1I positron activity by the proton bombardment of natural carbon is described. Extensi1.e subsicliary esperiments were performed to evaluate the errors in the factors entering the measurerneIlt. The cross section is (03.3 f 0.3) X lo-" cnlL per atom of natural carbon for protons of range 0515 mg/cm? of alurninum (indicated energy 08.1 MeV). INTRODUCTIOS + inch thick. Twelve different targets were used. From the quoted chemical *Present address: hT~~clear-Chicago Corporation, Des I'laines, Illinois. ?The cross section \\rill be reported here as though C1l decayed entirely by positron emission. Scobie and Lewis (1057) have reported (0.19 f 0.03)y0 I< capture.