  We present HST WFPC2 V band imaging for 23 Ultraluminous Infrared Galaxies
taken from the QDOT redshift survey. The fraction of sources observed to be
interacting is 87%. Most of the merging systems show a number of compact
`knots', whose colour and brightness differ substantially from their immediate
surroundings. Colour maps for nine of the objects show a non-uniform colour
structure. Features include blue regions located towards the centres of merging
systems which are likely to be areas of enhanced star formation, and compact
red regions which are likely to be dust shrouded starbursts or AGN. The host
galaxies of the QSOs in the sample were found to be either interacting systems
or ellipticals. Our data shows no evidence that ULIRGs are a simple transition
stage between galaxy mergers and QSOs. We propose an alternative model for
ULIRGs based on the morphologies in our sample and previous N-body simulations.
Under this model ULIRGs as a class are much more diverse than a simple
transition between galaxy merger and QSO. The evolution of IR power source and
merger morphology in ULIRGs is driven solely by the local environment and the
morphologies of the merger progenitors.
