Germanic acid was shown to inhibit photosynthetic 14C02 uptake in marine diatoms. Inhibition was not complete even at concentrations of 20 mg Ge/l nor in cultures incubated for extended periods of time (up to 24 h). The decrease in photosynthesis due to Ge(OH) 4 was independent of the stage of growth of the diatom culture. At 0.5 and 1.0 mg Ge/l, the degree of inhibition was dependent on the concentration of Si(OH) 4 in the medium. At 5 and IO mg Ge/l, inhibition was not affected by Si(OH) 4 concentrations as high as those found in the sea -120 Ng-at Si/l. The effect of Ge(OH) 4 on photosynthesis is specific for diatoms; other marine phytoplankton were not inhibited. In mixed cultures of diatoms and marine flagellates, the reduction in 14CO 2 fixation upon addition of Ge(OH) 4 was used to calculate the proportion of diatom photosynthesis to total photosynthesis, and calculated proportions agreed well with actual proportions. Inhibition by Ge(OH) 4 was also used to estimate the percent of diatom photosynthesis in a natural marine community, and this was compared with the diatom portion of the crop. Diatom photosynthesis was higher than one might expect from crop figures~ although both diatom photosynthesis and diatom numbers in the crop were low.