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 designated P-face and E-face. Each fracture faces has a population of particles which represent intramembrane proteins or protein complexes. 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 50,000X magnification.