Adsorption of a proteinlike heteropolymer is modeled at an oil/water interface by dynamic lattice Monte Carlo simulation. The heteropolymer is a designed sequence of 27 amino-acid-type lattice sites and has been used as a model for short (50-70) residue proteins. Oil is represented by a characteristic hydrophobic amino acid monomer, and water is represented by a characteristic hydrophilic amino acid monomer. The model protein is initially placed slightly away from the oil/water interface and is then allowed to undergo Verdier-Stockmeyer moves as amino acid sites interact with each other and with the oil and water. Local mixing of the oil and water is permitted over the length scale of the protein. Our lattice representation displays correct behavior in bulk water in that the model protein folds rapidly from an extended rod into a globular like state. In addition, there is a phase transition between the globular (folded) state and the denatured (unfolded) state at a particular temperature, T-m*. By examining the free-energy landscape at 0.94 T-m*, we identify four configurational states in the adsorbing system: unfolded in the bulk water, folded in the bulk water, unfolded at the interface, and folded at the interface. The most probable state of the four is the adsorbed unfolded state at the interface, with a large free-energy barrier to desorption. (similar to 20 k(B)T(m)*). We find that it is the unfavorable interaction between the oil and the water that drives the protein to the interface. Adsorption of a single protein molecule reduces the oil and water energies by 175 k(B)T(m)*. A typical conformation of the adsorbed, unfolded protein has the majority of protein segments remaining in the water but lying directly adjacent to the interface, with about 30% loops penetrating into the water phase and only a few segments (similar to 10%) penetrating into the oil. This work provides a picture of single-molecule protein adsorption at the oil/water interface in which the protein unfolds into an extended train structure and thereafter is essentially irreversibly bound.