SUMMARY The antibiotic powers of some common soil and leaf bacteria, streptomyces and moulds were tested against soil and leaf yeasts. Twelve of the seventeen bacteria used, ten of the eleven streptomyces strains and seven of the twenty-two moulds were inhibitory. Pseudomonas chlororaph.is, Aerornonas sp., five streptomyces and Tricho- derma viride inhibited 70 yo or more of the twenty-five yeast species used. The most frequently isolated soil yeasts were amongst those most sensitive to antibiotics. Only one species of leaf yeast but several soil yeasts were inhibited by leaf bacteria and moulds. As streptomyces do not occur on leaves, it seems that the wide seasonal fluctuations in kinds and numbers of yeasts on leaves are due rather to nutritional and physical factors than to antibiotic ones.