The adsorption of symmetric triblock copolymers from a nonselective solvent on an impenetrable surface has been studied by a lattice Monte Carlo simulation method. The triblock copolymers are A(NA)B(NB)A(NA) with N-A=10, 5 less than or equal to N-B less than or equal to 40, and surface interaction parameters -1.5 less than or equal to epsilon less than or equal to-0.5, where A is the adsorbing block and B is the nonadsorbing block. The concentration, expressed as the volume fraction, is varied over the range 0.012 less than or equal to c less than or equal to 0.143. This report describes the adsorption isotherms and the kinetics of the adsorption in;he simulations. At short times, the adsorption is diffusion controlled. Good approximations to the Langmuir adsorption isotherm are observed for the equilibrium adsorbed amount, Gamma and surface coverage of the adsorbing block, theta(A). The results are compared with recent simulations for diblock copolymers. The adsorbed amount is less for the diblock than for the triblock copolymers at low concentration. The surface coverage by the adsorbing block is the same for both cases at weak surface adsorption. The surface density profiles are also obtained and compared. The adsorbed number of chains per unit area (sigma) and surface coverage for triblock copolymers under : weak adsorption conditions are found, respectively, to scale according to the relationship sigma, theta(A) similar to 1/beta(2), where beta=(N-B/N-A)(3/5), which implies the importance of the nonadsorbing block size for the adsorption features of triblock copolymers. These results confirm the experimental observation reported recently by Dorgan et al. [Macromolecules 26, 5321 (1993)].