The French Space Agency (CNES) has developed a “wet” suit based on the Soviet flight suit. This concept, which requires a breathing system for underwater tests, has been fully validated by a replay of a real French-Soviet EVA space operation. The prototype offers the main advantage of placing both the astronaut and the suit in neutral buoyancy during EVA training.The man-machine interfaces were checked during several donning-doffing operations in simulations of weightlessness both in water and in parabolic flights. These experiments were analyzed from an ethological point of view to correlate the mechanical constraints of the suit with human motor performance.The ethological approach is characterized by the observation, description and quantification of the astronaut's behaviour when performing various tasks. Video recordings are made of his motor activity (orientation, movement and posture) in real or experimental situations such as water immersion or parabolic flights. The method consists of dividing the spontaneous body mobility into motor acts, listing them, and determining their occurrence frequency and sequential patterns.The following are examples of findings from our data. In parabolic flights, the diversification of orientations, regularity of motor patterns, decreased number of motor acts and minimal use of spatial volume for the donning/doffing of the EVA suit could be taken as behavioral indicators of a high level of performance by the astronauts. In water immersion, for the same task, the orientations and movements were less varied, the motor activity was more stereotyped and the subject used the water resistance for better body stability.In such a way, pplied ethology is an original tool for analyzing and quantifying human activities in space.