Four strains of Mycoplasma synoviae grown in broth medium were negatively stained and examined in the electron microscope. The morphology of all four strains was essentially similar to that of other mycoplasmas. Fixation of the mycoplasmas in hypertonic solutions resulted in extensive distortion of the organisms as did incubation for periods up to 50 h at 37 degrees C. No relationship could be demonstrated between the surface projections observed on the organisms and their ability to haemagglutinate chicken red blood cells.