LACATRE (Langage d'Aide a la Conception d'Applications Temps REel) is a graphical environment dedicated to multitasking real-time application developments. At the lowest level, it handles objects close to those supported by real-time executives (such as tasks, messages, semaphores ...), and at the highest level, it manipulates applicative objects (like agencies and processes), near to the application programmer's preoccupation. Graphical programming has been used in order to obtain a documentation with a high degree of legibility and a synthetic as well as precise view of the dynamic behaviour of a real-time application. The aim of this paper is to present the definition of a methodology assistant which takes advantage of the specificity of the Lacatre environment-behavioural axis, transformational axis, abstraction level axis, operative mode axis and graphical programming-, allowing an incremental development and providing an assistance adapted to the design context. Starting with a known, and rule based, stepwise design method, the methodology assistant can help the designer to verify the design steps and to monitor the complexity of the software components currently under construction.