Germanium dioxide (GeO 2 ) has been used for many years in the cultivation of red and green algae as a means of controlling the growth of diatoms. Brown algae are sensitive to GeO 2 , however, the basis of this sensitivity has not been characterized. Here we use embryos of Fucus vesiculosus to investigate morphological and physiological impacts of GeO 2 toxicity. Morphometric features of embryos were measured microscopically, and physiological features were determined using pulse amplitude modulated (PAM) fluorometry. At 5 mg L -1 GeO 2 , embryos grew slower than controls and developed growth abnormalities. After 24 h, initial zygote divisions were often oblique rather than transverse. Rhizoids had inflated tips in GeO 2 and were less branched, and apical hairs were deformed, with irregularly aligned, spheroidal cells. Minimum fluorescence (F 0 ) showed minor differences over the 10 days experiment, and pigment levels (chlorophylls a, c and total carotenoids) showed no difference after 10 days. Optimum quantum yield increased from ca. 0.52 at 24 h to 0.67 at 5 days, and GeO 2 -treated embryos had higher mean values (significant at 3 and 5 days). Optimum quantum yield of photosystem II (Φ PSII ) was stable in control thalli after 5 days, but declined significantly in GeO 2 . Addition of silica (as SiO 2 ) did not reverse the effects of GeO 2 . These results suggest that GeO 2 toxicity in brown algae is associated with negative impacts at the cytological level rather than metabolic impacts associated with photosynthesis.