  We report observations of deuterated acetylene (C$_2$HD) at 19.3 $\mu$m (519
cm$^{-1}$) with the Texas Echelon Cross Echelle Spectrograph on the NASA
Infrared Telescope Facility in July 2017. Six individual lines from the
Q-branch of the $\nu_4$ band were clearly detected with a S/N ratio up to 10.
Spectral intervals around 8.0 $\mu$m (745 cm$^{-1}$) and 13.4 $\mu$m (1247
cm$^{-1}$) containing acetylene (C$_2$H$_2$) and methane (CH$_4$) lines
respectively, were observed during the same run to constrain the disk-averaged
C$_2$H$_2$ abundance profile and temperature profile. Cassini observations with
the Composite Infrared Spectrometer (CIRS) were used to improve the flux
calibration and help to constrain the atmospheric model. The measured D/H ratio
in acetylene, derived from the C$_2$HD/C$_2$H$_2$ abundance ratio, is
(1.22$^{+0.27}_{-0.21})$ $\times$ 10$^{-4}$, consistent with that in methane
obtained in previous studies. Possible sources of fractionation at different
steps of the acetylene photochemistry are investigated.
