Aye, it's because Bryce 5 I believe uses filters to create it's effects, as opposed to actual geometry. This is also true with regular Boolean object groups... I noticed it first when I was working on this Mojoworld contest pic at home. If you take a sphere, hollow it with a negative one, set it to NOT transfer the neg material, and cut the sphere in half... well all of that, with a voumetric texture, the volume texture will render on the OUTSIDE of the sphere, but not on the holow inside. It sucks, but I'm creating a workaround. Maybe I should have posted it separately, sorry if it seems unrelated...