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As a prelude to future studies of molecular dynamics, the vibronic supersonic jet spectroscopy of the $S1(1B2u\leftarrow So(1Ag)$ transition of p-difluorobenzene-$d4$ (pDFB-$d4$) has been investigated for the lower 1600 $cm-1$ of the $S1$ vibrational manifold. Dispersed single vibronic level fluorescence spectroscopy has been used to establish 12 $S1$ fundamental frequencies. Significant spectroscopic differences from light pDFB occur. Two forhidden transitions not seen in the light molecule are observed, one being a second order Herzberg-Teller transition. Additionally, transitions with activity in a totally symmetric mode ($\nu 4$) that is inactive in light pDFB dominate the vibronic spectra of pDFB-d$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$. We explore with Franck-Condon analysis the reasons why this fundamental is not seen in the light molecule.""