Coplanar symmetric (e, 2e) results are presented for the ionization of calcium by electron impact. The incident electron energy was varied from 10.1 to 64.6 eV, the outgoing electrons sharing the excess energy equally. The experimental results show a complex variation in the magnitude of the cross-section as a function of energy and angle. At the highest incident energy, the cross-section varies by almost five orders of magnitude. At the lowest energy, evidence of post-collisional interaction between the outgoing electrons can be seen, although the magnitude of this effect is small. S This article has associated online supplementary data files