The negative kaon electromagnetic form factor has been measured in the space-like q2 range 0.015-0.10 (GeV/c)2 by the direct scattering of 250 GeV kaons from electrons at the CERN SPS. It is found that the kaon mean square charge radius <r2K> = 0.34 +/- 0.05 fm2. From data collected simultaneously for πe scattering, the difference between the charged pion and kaon mean square radii (which is less sensitive to systematic errors) is found to be <r2π> - <r2K = 0.1 0 +/- 0.045 fm2.
