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Using in-situ single crystal X-ray difiaction methods, thermally-and stress-induced crystal structure evolution was investigated in two Ni-Mn-Ga Heusler-type alloys. For the 5lat.O/oNi-24at.%Mn-25at.%Ga alloy it was found that application of external stress in a temperature range -20 OC above the M, at frst causes intensity changes of X-ray diffuse scattering peaks in P-phase. Further stressing results in stress-induced phase transformations and under the appropriate conditions three successive martensitic transformations (one is parent-tomartensite and two are martensite-to-martensite transformations) can be stress induced. Of these only the parent-tomartensite transformation can be thermally-induced. Two successive structural transformations (thermally-induced parent-to-martensite and stress-induced martensite-to-martensite transformations) were found in 52at.%Ni-25at.YoMn-23at.%Ga alloy. Crystal structure, lattice parameters, type of modulation, and the length of modulation period for all martensites were identified ' In the previous publications [3-51 this crysttal structure was refed as orthorhombic having a=P=y=90°

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