Materials whose structure or electrical or optical properties can be controlled with light are said to be photoactive. Liquid crystals are of interest as photoactive media, because their fluidity maintains the possibility of molecular motion in response to photon absorption, while their orientational and/or positional ordering offers the possibility of cooperative behaviour that can amplify relatively weak photochemical effects. Moreover, liquid crystals impose their own ordering on solutes. For example, smectic A liquid crystals, comprised of one-dimensional stacks of fluid layers with the molecular axes aligned normal to the layers＇, produce a modulation in solute concentration with a period equal to the layer spacing(2,3). Here we present computer simulations which show that the positional ordering of a photoactive solute tan azobenzene derivative, denoted 7AB) in a smectic host (denoted 8CB) depends sensitively on its photochemical state. The photoactive molecules are driven from within the smectic layers to locations between the layers by trans-to-cis photoisomerization, This would explain the recent observation(4-7) of a reversible increase in the smectic A layer spacing of a solution of 7AB in 8CB accompanying the photoisomerization process, The effect might be exploited for low-power, high-resolution optical data storage, and more generally for the manipulation of organic materials at the nanometre scale.