The thylakoids of the vegetative cells of the filamentous cyanobacteria, Oscillatoria tenuis, are capable of oxygen-evolving photosynthesis. Freeze-fracture replicas of the vegetative cells revealed the structure of the thylakoids. The process of freeze-fracture splits memebranes along their hydrophobic interior revealing two complementary fracture faces. Oscillatoria cultures were maintained on agar plates. Cells were collected and suspended in 10 mL dH2O, spun at 2,000 g for 10 min in a clinical centrifuge, resuspended in 5 mL dH2O. Added 5 mL 70% glycerol over 1 hr with continuous stirring, spun at 2,000 g for 10 min in a clinical centrifuge. Pellets were placed on copper support discs, frozen in liquid Freon 12 and stored under liquid nitrogen. Replicas were prepared on a Blazers BA 360 freeze-etch unit and examined in a JEOL TEM 100C at 16,000X magnification.