Dynamic mechanical properties of a symmetrical poly(ethylene-propylene) diblock copolymer with a molecular weight of 99 x 10(3) were investigated in the melt state, as well as in solutions in 1,2,4-trichlorobenzene. In the melt state, the frequency dependence of the elastic and loss moduli (G’ and G", respectively) exhibited a limiting behaviour of G’ approximate to G " approximate to omega(0.5) in the terminal zone, a marker of inhomogeneity. Even solutions of 15 vol% polymer showed such inhomogeneity. These theological data indicate that the diblock polymer is microphase separated and the phases are strongly segregated. The limiting behaviour in the terminal zone suggests that a lamellar structure is present in the melt.