  We present very deep HST/ACS images of five QSO host galaxies, classified as
undisturbed ellipticals in earlier studies. For four of the five objects, our
images reveal strong signs of interaction such as tidal tails, shells, and
other fine structure, suggesting that a large fraction of QSO host galaxies may
have experienced a relatively recent merger event. Our preliminary results for
a control sample of inactive elliptical galaxies do not reveal comparable fine
structure.
