The self-assembly and the adsorption of a diblock copolymer from a selective solvent have been simulated on a cubic lattice by a Monte Carlo method. We show that the self-assembly of the diblock copolymer greatly affects the adsorption. According to the adsorption behavior of the copolymer, the overall concentration can be divided into three regions. Below the cmc, the adsorption of the copolymer occurs from the free chains. The equilibrium mainly happens between the free chains and the adsorbed chains. Just above the cmc, the micelles have little contribution to the adsorption, and the adsorption of the copolymer also occurs from the free chains. There are two equilibria : one is the equilibrium between the free chains and the micelles, and the other, between the free chains and the adsorbed chains. The lower adsorbed amount just above the cmc is because the concentration of the free chains above the cmc is less than just below the cmc. At higher concentrations, the adsorption of the copolymer may occur from both the free chains and the micelles. There are three equilibria in the system, and the adsorbed amount is large because of the adsorption of the micelles. The simulation results of adsorption from a selective solvent are in good agreement with the experimental results. Below and just above the cmc the structures of the adsorbed layer in a selective solvent are similar to those in a nonselective solvent, and well above the cmc the segment density profiles have a different shape because of the self-assembly of the adsorbed chains.