  We observe a hyperfine anomaly in the measurement of the hyperfine splitting
of the 6S_{1/2} excited level in rubidium. We perform two step spectroscopy
using the 5S_{1/2}->5P_{1/2}->6S_{1/2} excitation sequence. We measure the
splitting of the 6S1/2 level and obtain for the magnetic dipole constants of
^{85}Rb and ^{87}Rb A = 239.18(4) MHz and A=807.66(8) MHz, respectively. The
hyperfine anomaly difference of_{87}delta_{85}=-0.0036(2) comes from the Bohr
Weisskopf effect: a correction to the point interaction between the finite
nuclear magnetization and the electrons, and agrees with that obtained in the
5S_{1/2} ground state.
