A Darboux type problem for a model hyperbolic equation of the third order with multiple characteristics is considered in the case of two independent variables. The Banach space The real number α 0 is found such that for α > α 0 the problem is solved uniquely and for α < α 0 it is normally solvable in Hausdorff's sense. In the class of uniqueness an estimate of the solution of the problem is obtained which ensures stability of the solution. 1991 Mathematics Subject Classification. 35L35.