ABSTRACT. The initial-boundary value problem associated with the motion of a Bingham fluid is considered.The existence and uniqueness of strong solution is proved under a certain assumption on the data. It is also shown that the solution exists globally in time when the data are small and that the solution converges to a periodic solution if the external force is time-periodic 0. Introduction.The purpose of this paper is to establish the existence of strong solutions to a variational inequality which describes the motion of a Bingham fluid in a bounded three dimensional domain. A Bingham fluid is a rigid viscoplastic fluid which is governed by a special constitutive law such that it moves like a rigid body if a certain function of the stresses does not reach the yield limit and it behaves like a viscous fluid when the yield limit is reached. Since the motion is governed by two entirely different stress-strain relations depending on the state of stresses, the conservation of momentum is expressed in terms of a variational inequality so that one can avoid the difficulty of separating the fluid zone and the rigid zone.The initial-boundary value problem we shall study is formulated asfor each test function w such that V ■ w = 0 in Yl and w = 0 on dYl,Here, fi is a bounded domain in R3 with smooth boundary dYl, u(x, t) denotes the velocity of the fluid and f(x, t) stands for external force. We assume that the density, the yield limit and the viscosity are positive constants. In particular, the