We use heavy element nucleosynthesis from supernovae to probe the mixing of νe with ντ (or νμ) possessing cosmologically significant masses (1 to 100 eV). We conclude that the ντ(νμ)-νe vacuum mixing angle must satisfy sin22θ<10-5, in order to ensure that r-process heavy elements can be produced in neutrino-heated supernova ejecta. Mixing at a level exceeding this limit precludes r-process nucleosynthesis in this site. © 1993 The American Physical Society.