The Δ15N of dissolved ammonium was determined in three anoxic marine basins: black Sea, Saanich Inlet, B.C., Canada, and Framvaren Fjord, Norway. In each basin, the Δ15N-NH4+ was greatest near the O2/H2S interface, with Δ15N as high as +21%. The depth distributions of NH4 + and Δ15N-NH4 + for black Sea and Framvaren Fjord were examined with a one-dimensional, steady-state, vertical advection-diffusion model to calculate the isotope fractionation during the consumption of NH4 + by bacteria. Isotope enrichments, ϵ, for Black Sea were between 5 and 15%, whereas in Framvaren Fjord ϵ ranged from 20 to 30%. These differences are related mainly to the ambient concentration of NH4 +. Biosynthetic uptake of NH4 + rather than nitrification was responsible for the fractionation. The Δ15N-NH4 + in Saanich Inlet appears related to in situ regeneration of NH4 + with little isotopic fractionation between dissolved and particulate nitrogen (PN). ¿ American Geophysical Union 1991