  The absolute frequency of the In$^{+}$ $5s^{2 1}S_{0}$ - $5s5p^{3}P_{0}$
clock transition at 237 nm was measured with an accuracy of 1.8 parts in
$10^{13}$. Using a phase-coherent frequency chain, we compared the $^{1}S_{0}$
- $^{3}P_{0}$ transition with a methane-stabilized He-Ne laser at 3.39 $\mu$m
which was calibrated against an atomic cesium fountain clock. A frequency gap
of 37 THz at the fourth harmonic of the He-Ne standard was bridged by a
frequency comb generated by a mode-locked femtosecond laser. The frequency of
the In$^{+}$ clock transition was found to be $1 267 402 452 899.92 (0.23)$
kHz, the accuracy being limited by the uncertainty of the He-Ne laser
reference. This represents an improvement in accuracy of more than 2 orders of
magnitude on previous measurements of the line and now stands as the most
accurate measurement of an optical transition in a single ion.
