The morphologies and the formation mechanisms of isothermally transformed Widmanstatten ferrite and bainite in various steels were investigated. Widmanstatten ferrite often grew from the grain boundary ferrite allotriomorph formed by a diffusional mechanism in the temperature range of the upper C-curve, whereas, bainite grew directly from an austenite grain boundary showing its own C-curve in a TIT diagram at temperatures between Bs (bainite starting temperature) and Ms (martensite starting temperature). Both structures accompanied well-defined surface reliefs of the invariant plane strain-type, and were in the shape of a lath or a plate consisting of several parallel needle-like ferrite subunits with parallelogram cross sc:ctions. The crystallographic properties of Widmanstatten ferrite and upper bainite were similar to those of lath martensite. Therefore, it was concluded that the difference between bainitic ferrite and Widmanstatten ferrite existed only in the nucleation events, where both structures grew by the same mechanism.