A simple model fluid, in which molecules consist of hard non-spherical cores embedded in spherical square-wells, has been re-examined. Within the Gaussian approximation of Onsager–Parsons–Lee theory, for very narrow attractive square-wells, ordinary liquid–vapour coexistence is preempted by a direct transition between a low density disordered fluid phase and a nematic phase. This is consistent with recent simulation results on fluids of Gay–Berne particles, and of hard spherocylinders with generalized square-wells which probed the effect of changing the ratio of attraction range to molecular elongation. Moreover, coexistence between two nematic phases is found on further decreasing the width of square-well, analogous to the isostructural solid–solid transitions observed in fluids characterized by very short range isotropic interactions.