Iridium, commonly used as a tracer of extraterrestrial material, was measured in rivers, oceans, and an estuarine environment. The concentration of iridium in the oceans ranges from 3.0 (+/-1.3) x 10(8) to 5.7 (+/-0.8) x 10(8) atoms per kilogram. Rivers contain from 17.4 (+/-0.9) x 10(8) to 92.9 (+/-2.2) x 10(8) atoms per kilogram and supply more dissolved iridium to the oceans than do extraterrestrial sources. In the Baltic Sea, similar to 75% of riverine iridium is removed from solution. Iron-manganese oxyhydroxides scavenge iridium under oxidizing conditions, but anoxic environments are not a major sink for iridium. The ocean residence time of iridium is between 2 x 10(3) and 2 x 10(4) years.