A photoionization source of monoenergetic electrons is described and design criteria for such sources are discussed. The present design produces a beam of ≃10−12–10−13A by photoionization of a metastable (1D2) barium beam inside the cavity of a He&sngbnd;Cd laser operating at 3250 Å. The photoelectrons are produced with 17 meV kinetic energy and a calculated energy spread of <1 meV. Energy analysis is provided by measuring the width of the 11.08 eV argon resonance. The observed width of this resonance has, to date, been limited to ∼6 meV, but tests are reported which indicate that this width is largely due to Doppler spreading in the target atomic beam and potential gradients across the collision volume.