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The rotational spectrum of Trifluoromethane-Ar ($CHF3$-Ar, see Fig. on the right) has been observed by free jet absorption millimeter wave spectroscopy. The near prolate top spectrum of the adduct suggests that: i) the location of Ar is out of the $C3v$ axis of $CHF3$; ii) the barrier to internal rotation of Ar is not negligible. The rotational lines are split in equally intense A E component lines for the low $K-1$ transitions, and in triplets lines for the high $K-1$, transitions (doubly overlapped because of the near prolate degeneracy) with intensities 2:1:1 for $A:E+:E-$ components. Such a behaviour is in accord with Hersbach theory for internal $rotation.a$. [FIGURE]