Li, Rb, and Cs were determined spectrographically in twenty-eight trilobites, two xiphosurida, four insects, four Crustacea, and six brachiopods. The sediments associated with fifteen trilobites were also analysed. Compared with igneous and sedimentary rocks, the trilobites were characteristically enriched in cesium relative to rubidium and potassium. Thus, the K\Rb ratios were normal, but the Rb\Cs ratios were anomalously low. The evidence is considered to favour the view that cesium has been preferentially concentrated by trilobites during growth. The matrices had Rb\Cs ratios similar to normal sedimentary rocks.