In this paper, I argue that perdurantism is incompatible with priority monism: the view that the universal mereological fusion, U, is fundamental. For the monist’s fundamental object can neither persist by being a trans-temporal object (i.e., a space-time ‘worm’) nor by being an instantaneous stage. If U persisted via being a worm, it would be grounded in its temporal parts, meaning that it would not be fundamental as it would not be ungrounded. If U were a stage, on the other (...) hand, it would face a problem from the possibility of ‘temporal gunk’. But if U persisted by neither being a worm nor a stage, then U could not persist via having temporal parts, and thus perdurantism would be false. Given that a similar combination of perdurantism and priority pluralism also faces a problem from temporal gunk, I conclude that perdurantism does not sit well with mereological based accounts of fundamentality. (shrink)