Microwave Detection ofH218O

Laboratory detection of a microwave absorption in ${\mathrm{H}}_2^{18}$O near 5.33 cm is reported. This observed signal has been assigned to a pure rotational transition between the $6_{16}$ and $5_{23}$ levels in the ground vibrational state of ${\mathrm{H}}_2^{18}$O. Signals from these same two rotational levels for ${\mathrm{H}}_2^{16}$O have been detected in emission from several sources in the galaxy.