  It is shown by computer simulations that superfluid {\it para}-hydrogen
clusters of more than 22 molecules can be turned insulating and "solidlike" by
the replacement of as few as one or two molecules, with ones of the heavier
{\it ortho}-deuterium isotope. A much smaller effect is observed with
substitutional {\it ortho}-hydrogen. Substitutional {\it ortho}-deuterium
molecules prevalently sit in the inner part of the cluster, whereas {\it
ortho}-hydrogen impurities reside primarily in the outer shell, near the
surface. Implications on the superfluidity of pure {\it para}-hydrogen clusters
are discussed.
