Three-dimensional distributions for 24- to 44.5 keV protons (ions) are presented from the Isee 1 medium-energy particles instrument during a magnetopause traversal at ~0145:00 hours UT on November 20, 1977. The use of these data and simple particle orbit geometry allows us to infer magnetopause boundary location, orientation, and velocity. The results are consistent with both overall boundary motions toward and away from the earth at velocities of 10--20 km/s and the presence of a surface wave on the boundary. For the case analyzed, the boundary appears stable and well defined, being capable of supporting trapped type distributions (conservation of the first and second adiabatic invariants) within a fraction (≲0.3&rgr;) of a gyroradius from the magnetosheath field where no trapping is observed.