The domain structure and phase separation in lipid single bilayers were observed directly by selective area electron diffraction (SAED) and selected reflection dark field electron microscopy (SRDFEM), using an environmental stage for a Siemens Elmiskop IA. The specimens were viewed in a fully hydrated state in the temperature range between 0°C and 50°C. Below the transition temperature, three orders of a hexagonal pattern were recorded, whereas diffuse rings only were seen above the transition temperature.Selective area electron diffraction were done by restricting the illumination area to few micrometers in diameter, using a pointed filament and a 10 μm second condenser aperture. Below the transition temperature of dipalmitoylphosphatidylchoiine bilayers, differentially oriented diffraction patterns were distinguishable between adjacent membranes areas (domains) five micrometers apart. Occasionally, two or more superimposed patterns were recorded from the same area, indicating the observation area covered several domains.