1. Chlorella vulgaris (Columbia strain) was subcultured in a boron-deficient medium contained in Corning boron-free flasks to bring the level of this element in the cells to a low level; such Chlorella cells were then inoculated into a medium containing 0, 0.5, 1, or 5 p.p.m. boron. 2. Chlorella cultures in a medium containing no boron but maintained in Pyrex flasks exhibited a greater concentration of cells after 24 days than did those grown in vessels of boron-free glass. This response undoubtedly resulted from a leaching of boron from Pyrex glass. 3. When compared with Chlorella cultured in boron-free medium, those in 0.5-p.p.m. boron exhibited a 60% increase in cell number and 137% increase in dry weight per cell. 4. The 1- and 5-p.p.m. boron levels resulted in less growth in comparison with the 0.5-p.p.m. cultures.