Abstract Nonlinear effects in resonant slow extraction from a synchrotron are such that a displacement in angle by means of a thin septum deflector can result in a displacement in position an integral number of betatron wavelengths downstream. This fact is exploited in the choice of location for an electrostatic septum to improve the extraction efficiency of the Brookhaven AGS. A curved septum, operating at 80 kV cm −1, is used. The extraction efficiency is measured by a loss monitor technique. The measured efficiency is 97%, compared with 85% without the electrostatic septum.