Among the published reports on invertebrate biochemistry is a study by Robin (1954) using Arenicola marina L. Robin hypothesized that taurocyamine, a sulfur-containing guanidine derivative, was formed from taurine. Although the hypothesis has not been demonstrated experimentally, it constitutes a major part of the information presently available on sulfur metabolism in invertebrates. Taurine in invertebrates generally has been regarded as a metabolically inactive compound. In Arenicola, however, since taurocyamine appears to be the phosphagen involved in energy transfers related to muscular activity, Robin's hypothesis requires an active metabolic role for taurine. Relatively high levels of taurine have been reported from a diversity of marine invertebrates. The fact that no definite role has been demonstrated for these high levels suggests that an organism, such as Arenicola, which presumably both synthesizes and metabolizes taurine, might be a useful experimental tool for studying sulfur metabolism in invertebrates. The present study was undertaken to determine the biochemical pathway leading to taurine formation in Arenicola cristata Stimpson, and to determine whether taurocyamine is synthesized by the route suggested by Robin or by some other mechanism.