Although pyrite is ubiquitous in fine-grained, organic, carbon-bearing marine sediments, it is only rarely involved in the preservation of soft-bodied organisms. Beecher's Trilobite Bed in Upper Ordovician strata of New York State is an exception—it is a classic locality for trilobites having appendages and other soft tissues preserved in pyrite. The relative timing and duration of the formation of pyrite associated with the fossils and their host sediments were determined by use of sulfur isotope ratios. The exoskeleton and appendages of the trilobites show relatively light sulfur isotope values in contrast to the enclosing sediment, which is characterized by a substantial excursion to heavy isotope values. Preservation of soft parts requires rapid burial of carcasses in sediments otherwise low in metabolizable organic matter. In these circumstances, pyrite formation within the sediments is suppressed; thus, concentrations of sulfate and reactive iron are initially high enough to promote early, rapid, and extensive pyritization of nonmineralized tissue.