Premieres observations sur le comportement de Thonius Braueri (thon, 1906) (Holothyrida). - Sorne behavioural aspects of Thonius braueri (Thon, 1906) are investigated in laboratory. Hexapody exists as in other Arachnids, the first pair of legs having a tactile function. Unlike Opilioacarids it is unable to escape backwards. The different kinds of food presented to them, in the rearing boxes have been refused except cheese and yolk. After walking or feeding this species brushes its chelicerae, its palps and its legs carefully. Classification and distribution of Holothyrida are mentioned. Then the air sacs function is discussed. This organ - called here Thon s organ - could have an excretory function and play a defensive role against predators.