Nonlinear emittance growth produced by ion extraction is considered by a 3‐D analysis in a Vlasov‐Poisson‐Boltzmann formulation. Phenomena considered include: (1), presheath effects, including electron depletion, (2), electron sheath accumulation (for large transverse magnetic fields), (3), nonlinear sheath fields (obtained by a self‐consistent solution with an assumed quasi‐equilibrium positive ion distribution and at least one Vlasov distribution), (4), nonlinear fringe fields produced by the accelerator‐extractor itself (obtained self‐consistently with item 3 above, (5), nonlinear space charge of the beam itself, (6), beam in conjunction with extracted electrons, and (7), ion acoustic waves. For specific volume negative ion source configurations, an investigation of the contribution of aberrations caused by an electron trap and electron accumulation in the extraction sheath are studied. Either of these effects can contribute significantly to the beam emittance, possibly dominating the contribution of the negative ion temperature in the source.