The proton and neutron radii of even-even beta-stable nuclei with A gr
eater than or equal to 40 and a few chains of isotopes with Z = 50, 56
, 82, 94 protons and isotones with N = 50, 82, 126 neutrons are analyz
ed. The average isospin dependence of the radii evaluated within the r
elativistic mean field theory is studied. A simple, phenomenological f
ormula for neutron radii is proposed. (C) 1998 Published by Elsevier S
cience B.V.