Angular distributions of + and - elastic scattering on Ca40 at 64.8 MeV are presented. The data are analyzed using a first-order, nonlocal optical model based on a Kisslinger t matrix with an off-shell form factor. This model is unable to fit the data for any value of the off-shell parameter ± unless the nuclear matter radius is allowed to vary from that derived from electron scattering. The resulting best-fit radii are as much as 0.40 fm larger than the electron scattering value for + and 0.55 fm larger for -. For a given ± the - best fit radius is also consistently larger by about 0.2 fm than that for +. In addition, the fits show no significant preference for any value of ± between 400 and 900 MeV/c. Inclusion of the normally omitted second-order potential terms in the optical model does not significantly alter the results. NUCLEAR REACTIONS + and - elastic scattering from Ca40 at 64.8 MeV; angular distributions, 15°<lab<127°; optical model analysis.