The quick-freeze, freeze-fracture, deep-etching and rotary replication techniques were used to analyze the structural organization of Giardia duodenalis. The surface of the flagella was rugous, in contrast to the cell body surface which was smooth. The ventral region was characterized in more detail, exposing the layer of sub-pellicular microtubules bonded to the microribbon sheet which appears as an open flat helicoid structure where the two free ends overlap and adhere close to the frontal part of the protozoan. The microribbon appears as 18-nm thick parallel (35 nm interval) filaments connected by short bridges. A flattened structure with a highly organized array of particles was seen close to the microribbons.