  We report the first detection in space of the single deuterated isotopologue
of methyldiacetylene, CH2DC4H. A total of 12 rotational transitions, with J =
8-12 and Ka = 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 allowed us to obtain, for the first time, the spectroscopic
parameters of this deuterated isotopologue. We derived a column density of
(5.5+/-0.2)e11 cm-2. The abundance ratio between CH3C4H and CH2DC4H is 24+/-2.
This ratio is similar to that found for the CH3C3N/CH2DC3N analogue system,
which is 22+/-2.We did not detect the deuterated species CH3C4D, which has
already been observed in laboratory experiments. The detection of deuterated
CH3C4H allows us to extend the discussion on the chemical mechanisms of
deuterium fractionation at work in TMC-1 using a new gas-phase chemical model
with multiply deuterated molecules. Introducing a possible deuterium exchange
reaction between CH3CCH and atomic deuterium allows us to account for the
CH3C4H/CH2DC4H abundance ratio.
