  We report the first detection in space of the single deuterated isotopologue
of methylcyanoacetylene, CH$_2$DC$_3$N. A total of fifteen rotational
transitions, with $J$ = 8-12 and $K_a$ = 0 and 1, were identified for this
species in TMC-1 in the 31.0-50.4 GHz range using the Yebes 40m radio
telescope. The observed frequencies were used to derive for the first time the
spectroscopic parameters of this deuterated isotopologue. We derive a column
density of $(8.0\pm 0.4) \times 10^{10}$ cm$^{-2}$. The abundance ratio between
CH$_3$C$_3$N and CH$_2$DC$_3$N is $\sim$22. We also theoretically computed the
principal spectroscopic constants of $^{13}$C isotopologues of CH$_3$C$_3$N and
CH$_3$C$_4$H and those of the deuterated isotopologues of CH$_3$C$_4$H for
which we could expect a similar degree of deuteration enhancement. However, we
have not detected either CH$_2$DC$_4$H nor CH$_3$C$_4$D nor any $^{13}$C
isotopologue. The different observed deuterium ratios in TMC-1 are reasonably
accounted for by a gas phase chemical model where the low temperature
conditions favor deuteron transfer through reactions with H$_2$D$^+$.
