Component of the peroxisomal and mitochondrial division machineries. Plays a role in promoting the fission of mitochondria and peroxisomes. In association with PEX11C, PEX11D, PEX11E and DRP3A, is involved in cell cycle-associated constitutive self-replication of preexisting peroxisomes. Interacts with PEX11A, PEX11B, PEX11C, PEX11D and PEX11E. The C-terminus is necessary for mitochondrial or peroxisomal targeting, while the N-terminus is necessary for mitochondrial or peroxisomal fission. Plants silencing FIS1B show reduced growth, increase in the size of mitochondria and decrease in the number of mitochondria per cell (PubMed:18785999). Overexpression of FIS1B increases the fission of peroxisomes and mitochondria (PubMed:19825601). Belongs to the FIS1 family.