Both mitochondria and peroxisomes fuse and divide dynamically, and key proteins of the fission apparatus localise to both organelles – but how are mitochondrial and peroxisomal fission regulated, and is regulation coordinated between the organelles? On page 3673, Agnès Delahodde and colleagues uncover a role for the proteasomal lid protein Rpn11 in peroxisomal fission in S. cerevisiae. The authors recently showed that mitochondria have a fragmented morphology in a strain of rpn11 mutant yeast (interestingly, this effect is independent of the proteasome's proteolytic activity). Now, they show that, under conditions that promote peroxisomal proliferation, peroxisomes are more abundant in rpn11 mutant yeast than in the wild type, and this is not a consequence of proteasome deficiency. Accordingly, an intact C-terminal domain of Rpn11, rather than the catalytic deubiquitinase domain, is necessary for regulating peroxisomal abundance and mitochondrial fragmentation. The authors identify Fis1 – which localises to both peroxisomes and mitochondria – as the fission-machinery component that is regulated by Rpn11. Finally, they show that Rpn11 co-purifies with both mitochondria and peroxisomes. These data help to clarify how organelle fission is regulated, and how the cell coordinates mitochondrial and peroxisomal morphology.