AN INVESTIGATION WAS UNDERTAKEN TO DETERMINE WHETHER ANY OF THE FOLLOWING FUNGI HAD A REQUIREMENT FOR BORON (B): Saccharomyces cerevisiae, Aspergillus niger, Neurospora crassa, and Penicillium chrysogenum. Boron was unessential, and hence a study was made of the concentrations of B that reduced the growth of S. cerevisiae and P. chrysogenum and the mode of action of the B toxicity. Fifty and 4000 mg B/liter, respectively, significantly (5% level) reduced the growth of the latter 2 species.In both, glycolysis appeared to be inhibited by toxic levels of B, since the cells accumulated fructose-1,6-diP and ADP, but were low in glyceraldehyde-3-P and ATP. With S. cerevisiae growing on glucose, 150 mg B/liter significantly reduced CO(2) evolution. When glyceraldehyde was substituted for glucose, CO(2) evolution and O(2) consumption were unaffected by this level of B.Aldolase was suspected of being inhibited by high B, and this was confirmed using a crude aldolase extract from S. cerevisiae and purified rabbit muscle aldolase. The inhibition of aldolase by B was uncompetitive.With aldolase activity being reduced by toxic levels of B, the fungi were apparently unable to utilize carbohydrates at a rate sufficient to maintain the metabolic processes involved in growth and reproduction.