This paper questions the relevance of the concept of « parts of the speech » regarding interjection. From the psychomecanical theory of word, Damourette and Pichon’s taxinomy and K. Mantchev’s structurology, the author proposes an original classification and a new definition of interjection as an element of substitution directly acting at the level of text (« énoncé »). Interjection must eventually be considered as a « part of the speech ».