SUMMARY1.Monascus rubervan Tieghem is reported found at the Botanical Museum of Harvard University growing on a specimen of smoked crude rubber ofHevea brasiliensisattacked by an undetermined species of mite.2.BothMonascus rubervan Tieghem andMonascus purpureusWent grew well on the nutriment offered them by the crude rubber apart from any material furnished by the mites.3.Crude rubbers from various plants served to varying degrees as utilizable media for growth of both species ofMonascus.4.Monascus ruberdeveloped somewhat better thanMonascus purpureuson unvulcanized samples of commercial rubber: pale crêpe, smoked sheet, and latex-sprayed rubber. Pale crêpe is the least suitable of these as a nutrient substratum. The smoked condition of the smoked sheet seemed to hinder but little the support of the fungi. No growth of either species was produced on the vulcanized samples.5.Pure rubber hydrocarbon representing the sol-rubber portion of the hydrocarbon constituent supported growths of both species ofMonascus.6.Synthetic rubber, a polymer of chloroprene, was not utilizable as a substrate by eitherMonascus ruberorMonascus purpureus.7.Monascus, a cosmopolitan genus found on a variety of substrata may become an important member of the group of fungi attacking rubber.