This study reports on the isotopic, trace- and minor-elemental analysis of some Ordovician and Devonian trilobite cuticles from Ontario, Canada, and New York State, U.S.A. The Devonian trilobite Phacops rana and the Ordovician trilobite Isotelus gigas, obtained from shale and limestone, constitute most of the studied specimens. Specimens from shales are generally better preserved than their counterparts from limestones. Trilobites from shales exhibit the highest Sr, Na, Mg, Mn and Fe contents, but diagenetic evaluation has to be carried out on individual populations to select the best-preserved specimen(s). In well-preserved specimens, there is no significant change in geochemical content due to maturity, location of sampling within the cuticle, species or age of the specimen. Therefore, fragments of a cuticle are representative of an entire specimen and the elemental contents of the cuticular calcite reflect homogeneous body fluid composition during deposition. In comparison with originally low-Mg calcite shells of brachiopods and high-Mg calcite skeletons of crinoids, the studied trilobites possessed cuticles of an intermediate-Mg calcite composition. It is postulated that the Mg content varied from ∼2 to ∼7 mole% MgCO 3. This intermediate-Mg calcite composition for trilobites complements the results of studies on the calcite fraction of many modern marine arthropods. Isotopic compositions of well-preserved trilobites are similar to those obtained from contemporaneous brachiopods. Oxygen and carbon isotope values of altered specimens suggest that marine-meteoric mixed water was the predominant diagenetic fluid.