A sensitive spectrophotometric method using phenylfluorone has been applied to the determination of germanium in over 200 minerals, rocks and sediments and in sea water. The crustal abundance of germanium is about 1.7 p.p.m., which is the mean germanium content of both granitic and basic igneous rocks. The average amounts of germanium in metamorphic rocks, shales and red clays lie close to this value. The germanium content of pelagic sediments is contained entirely in their clay fraction. Of the minerals examined, sulphates, carbonates and evaporite minerals contained only very low concentrations of the element. Most oxides and silicates have germanium contents close to the mean crustal abundance. The average concentration of germanium in magmatic sulphides is less than 1 p.p.m. Sea water has been found to contain 0.06 ± 0.01 μg Ge/kg (0.82 ± 0.13 μg- atom/ ton). Gallium has been determined in sea water and in over 280 minerals, rocks and sediments using a sensitive and specific spectrophotometric method. The average crustal abundance of the element is 16.9 p.p.m. The gallium content of over 80 per cent of the thirty-three granitic rocks examined lay between 14 and 24 p.p.m. (average 18.1 p.p.m.). The average Ga: Al ratio for such rocks is 2.35 × 10 -4. The mean gallium concentration found for twenty siliceous sediments is 9.9 p.p.m., in general, residual sandstones and other sedimentary rocks rich in silica are poor in gallium. Hydrolysate sediments such as clays and mudstones are similar to granitic rocks in gallium content. Only traces of gallium are found in carbonate rocks (mean 0.06 p.p.m.). Pelagic clays from the Pacific, Atlantic and Indian Oceans contained an average of 22.4 p.p.m. of the element and had a Ga: Al ratio of 2.40 × 10 -4. Most of the gallium present in the other pelagic sediments is contained in their clay fraction. The element is enriched relative to aluminium in manganese nodules (mean Ga: Al = 9.7 × 10 -4). The only oxide minerals examined which contain noteworthy amounts of gallium are corundum, bauxite and magnetite. With the exception of blende formed at low temperature all the sulphides investigated contained less than 3 p.p.m. of the element. Carbonate, sulphate, fluoride and evaporite minerals generally had not more than 0.15 p.p.m. Gallium is concentrated in felspars of pegmatite origin and in the late crystallizates, such as micas. Sea water contains 0.030 ± 0.007 μg of gallium/kg (0.44 ± 0.10 μg- atom/ ton) and has a Ga: Al ratio of about 3.0 × 10 -3.