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Spectra with a high signal-to-noise ratio of 50 stars which are just belowthe main sequence turnoff and are members of M13 are presented. They yieldindices for the strength of the CH and the ultraviolet CN band. There is noevidence for a variation in the strength of either feature from star to star inthis intermediate-metallicity galactic globular cluster, and thus no evidencefor primordial variations in the abundance of C and N in M13. This supports thehypothesis that abundance variations found among the light elements in theevolved stars of M13 by Suntzeff (1981), and commonly seen on the giant andsubgiant branches of globular clusters of comparable metallicity, are dueprimarily or entirely to mixing within a fraction of individual stars as theyevolve.Comment: Accepted for publication in the AJ, to appear back to back with paper  II, 15 pages with 5 figure

The Spectra of Main-Sequence Stars in Galactic Globular Clusters. I. CH and CN Bands in M13