We study the phase coexistence in random copolymer systems. In the framework of the one-loop approximation it is shown that the phase transition between liquidlike and lamellar phases in the symmetric random copolymers occurs through the two-phase region. We derive an analytic expression describing the volume fi-actions of the coexisting phases in this region, To illustrate the phase behavior of the random copolymers, the phase diagram of a polydisperse A-B multiblock copolymer melt is calculated. (C) 1997 American Institute of Physics.