We conducted deformation experiments to investigate the strength, deformation processes, and nature of the brittle-ductile transition of lizardite and antigorite serpentinites. A transition from localized to distributed deformation occurs as confining pressure increases from ~200 to ~400 MPa at room temperature. Deformation in both brittle (localized) and ductile (distributed) regimes is accommodated by shear microcracks, which form preferentially parallel to the (001) cleavage. Axial microcracks (mode I) are infrequently observed. Volumetric strain measurements demonstrate that brittle deformation is mostly nondilatant, consistent with the shear-dominated microcracking. Three observations indicate that deformation in the ductile regime is accommodated by cataclastic flow: (1) a lack of evidence for crystal plastic deformation, (2) a positive pressure dependence of the maximum differential stress, and (3) abundant evidence for brittle microcracking. The weakness of serpentinites relative to other brittle rocks is explained by a low fracture strength along the (001) cleavage, combined with the low pressure dependence of strength. The transition from brittle to ductile deformation occurs at the crossover between the strength of intact serpentinite and the friction law unique to each type of serpentinite, rather than the more general Byerlee's law. If brittle deformation regimes are defined based on the mode of microcracking and on the occurrence of crystal plasticity, serpentinites define an end-member style of nondilatant brittle deformation. This deformation style may result in extremely weak faults in nature, and it may also strongly influence the tectonic evolution of the oceanic lithosphere where serpentinite is present.¿ 1997 American Geophysical Union