In this paper we investigate the phase behavior of a "simple" fluid confined to a chemically heterogeneous slit pore of nanoscopic width s(z) by means of Monte Carlo simulations in the grand canonical ensemble. The fluid-substrate interaction is purely repulsive except for elliptic regions of semiaxes A and B attracting fluid molecules. On account of the interplay between confinement (i.e., s(z)) and chemical decoration, three fluid phases are thermodynamically permissible, namely, gaslike and liquidlike phases and a "bridge phase" where the molecules are preferentially adsorbed by the attractive elliptic patterns and span the gap between the opposite substrate surfaces. Because of their lack of cylindrical symmetry, bridge phases can be exposed to a torsional strain 0less than or equal tothetaless than or equal topi/2 by rotating the upper substrate while holding the lower one in position. Depending on the thermodynamic state of the confined fluid, torsion-induced first-order phase transitions are feasible during which a bridge phase may be transformed into either a gaslike (evaporation) or a liquidlike phase (condensation). Since the chemical patterns decorating the substrates are finite in size, system properties are not translationally invariant in any spatial direction. Therefore, in order to study these phase transitions, we resorted to the thermodynamic integration scheme developed earlier to calculate the grand potential Omega in a system of low symmetry. (C) 2004 American Institute of Physics.