Isotopes of $^{123\ensuremath{-}137}\mathrm{Cs}$ produced by spallation of lanthanum and separated in mass by the ISOLDE on-line facility have been transformed into an atomic beam which is illuminated with a tunable cw dye laser. From the sensitive detection of the optical resonance lines at 459 nm, the hyperfine structure of $^{123\ensuremath{-}132}\mathrm{Cs}$, $^{137}\mathrm{Cs}$, and $^{130m}\mathrm{Cs}$ has been determined. The interpretation of isotope shifts, in terms of variation of charge radii, is discussed.