In enriched sea-water cultures inoculated with a mixture of algae and other organisms, concentrations of GeO2 above 1.5 mg/l specifically suppressed the growth of diatoms. In experiments with 14 pure cultures of diatoms (10 species), 1 mg GeO2/l sufficed to reduce the growth rates significantly; 10 mg GeO2/l was even more inhibitory to growth, and in a few cases killed the cells. Phaeodactylum tricornutum, the most weakly silicified diatom, was the least sensitive to GeO2 inhibition. In experiments with four selected marine species, an appropriate increase in the SiO2 concentration of the medium reversed the inhibitory action of GeO2 on growth. A similar effect of SiO2 was observed in growth experiments with the fresh-water diatom Navicula pelliculosa. (A non-silicified green fiagellate responded quite differently, its growth being unaffected by relatively high concentrations of GeO2.) At these low concentrations, GeO2 presumably had no effect on respiratory metabolism, since, even in concentrations of GeO2 as high as 400 mg/l, oxygen uptake of N. pelliculosa was unimpaired. All of these experimental results are consistent with the hypothesis that GeO2 is a specific inhibitor of silicate utilization.