Celestite nodules with subordinate amounts of chalcedony are common in Upper Cretaceous chalk in two cores drilled at Stevns, Denmark. The nodules occur throughout the cores, indicating that the environmental conditions that led to their formation were an integral part of the biogeochemical cycle in the chalk sea. Petrographic evidence indicates that the celestite nodules were formed by replacement of lithistid demosponges in localized reduced environments prior to sediment compaction and chert formation. The 87Sr/86Sr values of the celestite nodules indicate that the Sr is mainly sourced from seawater and the δ34S values indicate activity of sulfate-reducing bacteria. This unusual occurrence of celestite is interpreted as a result of rapid burial of the sponges and following decomposition of the organic tissue by sulfate-reducing and sulfide-oxidizing bacteria, forming an anoxic microenvironment. The decomposition produced hydrogen sulfide, which was oxidized to sulfate by sulfide-oxidizing bacteria, resulting in decreasing pH. Strontium precipitated as celestite in dysoxic microenvironments and at the anoxic-dysoxic interface within the sponge. The crystallization of celestite apparently created an Sr gradient that drove Sr into the site of crystallization. This geochemical environment in dead demosponges represents an unknown celestite-forming process.