A trans-Sasakian structure is, in some sense, an analogue of a locally conformal Kähler structure on an almost Hermitian manifold. Two remarkable subclasses of trans-Sasakian structures are those called \({\mathcal C}_ 5\)- and \({\mathcal C}_ 6\)-structures, which contain the Kenmotsu and Sasakian structures respectively. In this paper the author has completely characterized the local nature of trans-Sasakian structures on differentiable manifolds of dimension \(\geq 5\). This has been done through two stages: 1) characterizing the local nature of \({\mathcal C}_ 5\)- and \({\mathcal C}_ 6\)-structures; 2) showing that a trans- Sasakian structure is either of class \({\mathcal C}_ 5\) or of class \({\mathcal C}_ 6\). The author has finally obtained some examples of 3-dimensional trans-Sasakian manifolds which are neither of class \({\mathcal C}_ 5\) nor of class \({\mathcal C}_ 6\).