Microindentation at room temperature has been studied, by transmission electron microscopy, for GaAlAs and GaInAs with a range of compositions. Both p-type and n-type alloys have been investigated with particular reference to the formation of microtwins. A change of doping from n-type to p-type in GaAlAs results in microtwin formation along perpendicular ⟨110⟩ rosette arms. This is also observed in n-type GaInAs alloys.