Geodetic and seismic data collected between July 1982 and August 1981 document an important event in the recent evolution of Long Valley caldera. In January 1983 a substantial change in the pattern of deformation was associated with the most intense seismic swarm since May 1980. The earthquake hypocenters of the January 1983 swarm formed a vertical, roughly tabular zone beneath the south moat of the caldera. Analysis of extensive intracaldera horizontal control data indicates that the shear strain distributed throughout this zone could not all be attributed to slip along any single fault plane. The surface deformation of the resurgent dome and caldera floor is temporally associated with the swarm and was caused by an expansion of the upper levels of the residual magma chamber; analysis of the 1983 data indicates a pressure distribution 2--5 km shallower than could be fit by analysis of the 1979--1982 caldera deformation. The volume of this shallow expansion is 10--15% of the whole chamber expansion that occurred between 1979 and 1982. Most of the 1983 deformation occurred between December 14, 1982, and January 20, 1983, and thus was associated with the January 6--7, 1983, swarm, though right-lateral shear strain continued to accumulate in the south moat through 1983. We conclude that a shallow-pressure increase probably occurred within the upper levels of the residual magma chamber of Long Valley caldera during the 5-week period following December 14, 1982. It is not clear, however, whether this pressure increase preceded or accompanied the January 1983 earthquake swarm.