Polyhalogenated 1'-methyl-1,2'-bipyrroles are natural products that biomagnify into upper trophic levels of marine food webs. Here we demonstrate that they are unusually enriched in (15)N (?(15)N from +19.3‰ to +28.1‰) relative to other biosynthetic organic compounds measured to date and the mammals from which the compounds were isolated. We argue the (15)N enrichment likely stems from enriched precursors and/or fractionation during biosynthesis and is not from MBP degradation. We also consider possible sources of MBPs in light of these results.