  The only alkali metal known to be superconducting at ambient pressure is Li
at 0.4 mK. Under 30 GPa pressure \textit{T}$_{c}$ for Li rises to 14 K. In
addition, nearly 50 years ago the heavy alkali metal Cs was reported to become
superconducting near 1.3 K at 12 GPa. In the present experiment the
superconductivity of Cs under pressure is confirmed. In addition, strong
evidence is presented in electrical resistivity measurements that neighboring
Rb also becomes superconducting near 2 K at 55 GPa as it enters the
\textit{oC}16 phase, as for Cs, where \textit{T}$_{c}$ decreases under the
application of pressure. It would seem likely that under the right
temperature/pressure conditions all alkali metals, including metallic hydrogen,
will join the ranks of the superconducting elements. With the addition of Rb,
55 of the 92 naturally occurring elements are superconducting at ambient or
high pressure.
