We report grand canonical Monte Carlo (GCMC) molecular simulation studies of simple fluid adsorption in buckytubes. Buckytubes are graphitic tubes with internal diameters of 1-5 nm and a regular pore structure. Adsorption isotherms and isosteric heats of adsorption are shown for argon and nitrogen in a microporous buckytube at 77 K, for argon and nitrogen in a mesoporous buckytube at 77 K, and for argon in a mesoporous buckytube at 55 K. The smaller buckytube shows type I adsorption behavior. Layering and hysteresis (type VI and IV adsorptions, respectively) are observed for the larger buckytube. A temperature of 77 K is shown to be below the critical temperature for capillary condensation and above the critical temperature for layering transitions for both nitrogen and argon adsorption. A temperature of 55 K is shown to be below the critical temperature for layering transitions for argon adsorption. Argon adsorption at 77 K in an open-ended mesoporous model buckytube is also presented. A comparison is made with argon adsorption at 77 K in a "semiinfinite" model buckytube of the same diameter. The open-ended model gives a narrower, more rounded hysteresis loop.