Acid decomposition with nitric, perchloric and/or hydrofluoric acids is used to extract selenium from marine geological materials. Selenium is isolated from potentially interfering elements by coprecipitation with lanthanum hydroxide, then a piazselenol is formed between 2, 3-diaminonaphthalene at pH 1.0 and extracted into cyclohexane. The selenium is determined fluorimetrically in the extract. The detection limit is 3.2 ng (four times the standard deviation of blank) and the coefficient of variation is 5% at the 10 ng level. The sample decomposition procedure is evaluated and typical results obtained for selected samples are given.