Siliceous and calcareous biogenic rocks host pervasive, although volumetrically minor, authigenic barite on Lefkas Island, Greece. The barite composes up to 10 percent of the carbonate-free fraction of samples. In decreasing order of abundance, authigenic barite occurs as infilled foraminifera and radiolarian tests, disseminated grains and aggregates in porcelanite and siliceous limestone, replaced siliceous and calcareous biogenic tests, and thin laminae and lenses. The laminae form anastomosing networks that were probably originally rich in organic matter. Barite abundance shows a good correlation with the abundance of skeletal biogenic components. Biogenic silica diagenesis produced opal-CT and clinoptilolite, which compose the cement of porcelanite, replaced radiolarians, diatoms, and echinoderms and filled in some foraminifera tests. Subsequent silica diagenesis produced chalcedony at deeper stratigraphic levels, which also filled in pore space and calcareous microfossils in siliceous limestones and formed chert beds. Textural evidence shows that barite mineralization postdated opal-CT formation and preceded chalcedony formation. Barite precipitated in microenvironments including microfossil chambers, fecal pellets, and areas rich in organic matter. The source of the barium was probably from the decomposition of organic matter derived from plankton and bacteria and from the diagenesis of silica and carbonate tests. The source of the sulfate ion was seawater; possibly additional sulfate was produced and utilized in microenvironments from degradation of organic matter.