A physical model of the mature pox virus particle is given by calculated surfaces of constant potential as found in some electrostatic or other fields and is compared with electron micrographs of thin sections of mature canary pox virus. It is built up by selected surfaces of constant potential V=1/r1+1/r2+1/r3 +1/r4 = constant where r1, r2, r3 and r4 stand for the distances between a point in a surface and four potential generating centers arranged in the corners of a square or a rectangle. The possible biophysical reasons for the high similarity with the morphological features are discussed.