Monte Carlo simulations were performed with self- and mutually-avoiding symmetric diblock chains on a cubic lattice with periodic boundary conditions. The concentration of diblock copolymer was varied to cover a wide range of volume fraction, 0.45-0.85. The empty lattice sites were treated as a selective solvent, so that the insoluble blocks were seen to aggregate into various morphologies. Spherical, cylindrical, monocontinuous catenoid-lamellar (mesh) and lamellar morphologies were observed as a function of increasing concentration, in that order. The cylindrical morphology changed to monocontinuous catenoid-lamellar morphology by adhesion of nearest-neighbor cylinders, as the concentration of diblock copolymer increases. The concentration at which the transition between morphologies occurs can be identified by analysis of the anisotropy of the translational diffusion coefficient.