In the polar bear food chain from the Canadian Arctic, methyl sulfone (MeSO2-) PCBs and 4,4‘-DDE were below detection in arctic cod (<0.01 ng/g, lipid wt). Ringed seal blubber contained 3-MeSO2-4,4‘-DDE (0.4 ng/g) and 14 3- and 4-MeSO2-PCB isomer pairs (∑MeSO2-PCB, ca. 13 ng/g) formed by the biotransformation of PCBs not chlorine substituted at the meta−para positions on one ring (m,p-PCBs). Bioaccumulation/formation efficiencies relative to CB153 (BFE-) from cod to seal were 0.001−0.086 for MeSO2-PCBs and 0.004 for 3-MeSO2-4,4‘-DDE. Twelve MeSO2-PCB isomer pairs (∑MeSO2-PCBs, 432 ± 57 ng/g) and 3-MeSO2-4,4‘-DDE (2.0 ± 0.7 ng/g) were identified in polar bear fat; BFE‘ values were 0.03−0.62 and 0.0001 for MeSO2-PCBs and 3-MeSO2-4,4‘-DDE, respectively. Methyl sulfone formation is important but not the major route for m,p-PCB and 4,4‘-DDE biotransformation in polar bear and ringed seal. Fifteen MeSO2-PCB congeners in the bear are likely bioaccumulated from seal relative to the completely bioaccumulated 3-/4-MeSO2-CB132. Strong evidence exists for the partial formation of seven MeSO2-PCBs in the bear. We conclude that MeSO2-PCBs have high biomagnification potential in food chains.