The Ploumanac'h massif in Brittany is a funnelshaped, late-orogenic granitic diapir. Strain associated with the emplacement of this diapir is analysed with reference to the structure of the envelope, the form of the mafic schlieren and enclaves and fracture patterns in the coarsegrained granites. The behaviour of the country rocks was first brittle, then plastic. In the granites successive deformations seem to have been more or less continuous: viscous flow—identified in layered schlieren and suspected elsewhere—gave way to plastic flow and cataclasis as crystallization developed. The viscosity of the surrounding rocks decreased as the viscosity of the crystallizing magmas increased. The form of the intrusion and its changing mechanism of emplacement are related to this decrease in viscosity contrast between the granite and its envelope.