Preexponentials ${D}_{0}$ and activation energies Q for self-diffusion and diffusion of homovalent cations of small radius in single crystals of CsI measured by the tracer sectioning method are as follows: for $^{137}\mathrm{Cs}$, 19\ifmmode\pm\else\textpm\fi{}1.5 ${\mathrm{cm}}^{2}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ and 35.59\ifmmode\pm\else\textpm\fi{}0.18 kcal/mol; for $^{86}\mathrm{Rb}$, 30.7\ifmmode\pm\else\textpm\fi{}3.0 ${\mathrm{cm}}^{2}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ and 35.13\ifmmode\pm\else\textpm\fi{}0.22 kcal/mol; for $^{42}\mathrm{K}$, 47\ifmmode\pm\else\textpm\fi{}6 ${\mathrm{cm}}^{2}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ and 35.30\ifmmode\pm\else\textpm\fi{}0.30 kcal/mol; for $^{22}\mathrm{Na}$, 34.94\ifmmode\pm\else\textpm\fi{}0.20 kcal/mol, respectively. The temperature dependence of the isotope effect for diffusion of $^{22}$,24Na in CsI is described by a simple exponent: E=126 exp[-(13.3\ifmmode\pm\else\textpm\fi{}1.5 kcal/mol)/RT]. Calculations based on these experimental results and the ``five-frequency model'' of diffusion are provided to obtain the main diffusion parameters for all homovalent cations in CsI. The diffusion behavior of these cations in CsI is in good agreement with the model of ``displaced homovalent cations of small radius.''