An approach is presented to describe the mechanical behaviour of shape memory alloys undergoing a fcc - hcp phase transformation. Previously encouraging results have been obtained within the frames of the structure-analytical theory [1]. They were achieved by introducing specific limitations on the maximum allowable size of martensite and austenite crystals. The model described in this report differs by the averaging procedure which is organised in a way to account for the symmetry of fcc and hcp lattices as well as the symmetry of the transformation strain tensor. The results of modelling have shown that symmetrical considerations can explain the transformation plasticity (strain accumulation due to a transformation in a stressed specimen) while direct or reverse transformations, the incompleteness of the strain recovery in the subsequent reverse or direct transformation and the asymmetry of the mechanical behaviour for these two transformations. A qualitative agreement whith experiments on Fe-Mn alloys (except for the scale of the asymmetry between direct and reverse transformations) has been acheived, the errors of modelling being probably connected with the dislocation plasticity not accounted for in the model.