Abstract. We study initial-boundary value problem for an equation of composite type in 3-D multiply connected domain. This equation governs nonsteady inertial waves in rotating fluids. The solution of the problem is obtained in the form of dynamic potentials, which density obeys the uniquely solvable integral equation. Thereby the existence theorem is proved. Besides, the uniqueness of the solution is studied. All results hold for interior domains and for exterior domains with appropriate conditions at infinity. Modern advances in the theory of waves are mostly concerned with nonlinear phenomena [4,3,5,6]. However, there exist some types of linear waves which are not well studied yet, for example, nonsteady inertial waves, that is, internal waves in rotating fluids. These waves are governed by PDEs of composite type and of high order. Such PDEs possess both elliptic and hyperbolic characteristics, and so they share properties of both elliptic and hyperbolic equations [13,23,24,25,26]. Equations of composite type are not well studied even in linear case and they are not covered by existing classifications of PDEs. In particular, they do not belong to equations of principal part, though elliptic, parabolic, and hyperbolic equations belong [7].Inertial wave equation (see (1.2)) is an evolutionary equation of fourth order and composite type. This equation comes from ocean dynamics. Different initial-boundary value problems for analogous equations were treated in [8,9,10,14,15,16,17,18,19,20,30]. In particular, 2-D problems in multiply connected domains were studied in [15,16,17,18,19,20]. Initial boundary value problems in simply connected domains were studied for 3-D equation of inertial waves and for similar equations in [8,9,10,14,30], but 3-D problems were not treated in multiply connected domains before. The aim of the present paper is to consider initial-boundary value problem in 3-D multiply connected domain (interior and exterior) with Dirichlet boundary condition. To prove existence theorem for our problem we use dynamic potentials [14] and the method of boundary integral equations. Uniqueness of a solution is also studied. It should be stressed that the integral equation obtained in the present paper is a uniquely solvable Fredholm equation of the second kind and index zero. This equation can be easily computed by discretization and inversion of a matrix. Initial-boundary value problems for some equations of composite type in a 3-D simply connected domain were reduced to integral equations in [8,10,14]. However, the method of reduction to integral equations suggested in these papers for simply connected do-