Pyrite within and below sapropels in the eastern Mediterranean is a result of microbial SO42− reduction within the sapropel, and the subsequent reaction of sulfide (HS−) with detrital Fe and Fe2+ diffusing upward from underlying sediments. Below the youngest Mediterranean sapropel, S1, pyrite (as much as 281 µmol pyritic S/g) is mostly present as euhedral crystals, whereas within the sapropel only framboidal pyrite (as much as 360 µmol pyritic S/g) has been detected. Framboidal microtextures indicate pyrite formation at the site of HS− production within the sapropel. Euhedral pyrite, below the sapropel, forms when sulfate reduction in the sapropel outcompetes iron liberation and supply, and HS− diffuses out of the sapropel. Sulfur isotope values of pyrite are extremely light in the sapropel (−37.3‰ to −38.2‰) as well as below the sapropel (−45.6‰ and −49.6‰), indicating that HS− has formed in a system with abundant SO42− and in the presence of oxidants.