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 mu mol pyritic S/g) is mostly present as euhedral crystals, whereas within the sapropel only framboidal pyrite (as much as 360 mu mol 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 parts per thousand to -38.2 parts per thousand) as well as below the sapropel (-45.6 parts per thousand and -49.6 parts per thousand), indicating that HS- has formed in a system with abundant SO42- and in the presence of oxidants. [KEYWORDS: Laboratory synthesis; marine-sediments] ; 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 mu mol pyritic S/g) is mostly present as euhedral crystals, whereas within the sapropel only framboidal pyrite (as much as 360 mu mol 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 parts per thousand to -38.2 parts per thousand) as well as below the sapropel (-45.6 parts per thousand and -49.6 parts per thousand), indicating that HS- has formed in a system with abundant SO42- and in the presence of oxidants. [KEYWORDS: Laboratory synthesis; marine-sediments]