Deuterium ignites within protostars approaching the main sequence before significant hydrogen buming occurs. If the interstellar gas from which stars are formed has a sufficiently high abundance of deuterium, young clusters will contain low-maas stars still on the deuterium main sequence,'' 3-4 mag above the hydrogen main sequence, The absence of such stars in the Pleiades cluster limits the deuterium abundance to D/H < l.5xl0/sup 3/ by number of atoms. Improved observations can lower this limit by more than a factor of 2. (auth)