  We present a measurement of the hyperfine splitting of the 6S$_{1/2}$ excited
level of rubidium using two photon absorption spectroscopy in a glass cell. The
values we obtain for the magnetic dipole constant A are 239.18(03) MHz and
807.66(08) MHz for $^{85}$Rb and $^{87}$Rb, respectively. The combination of
the magnetic moments of the two isotopes and our measurements show a hyperfine
anomaly in this atomic excited state. The observed hyperfine anomaly difference
has a value of $_{87}\delta_{85}=-0.0036(2)$ due to the finite distribution of
nuclear magnetization, the Bohr-Weisskopf effect.
