Propargyl (HCCCH(2)) radicals have been generated by expanding precursors through a supersonic pyrolysis nozzle, and their infrared (IR) absorption spectra have been recorded in a rare gas matrix. Besides the 10 vibrational modes observed in our previous studies, this investigation detected the HCCCH(2) X̃ (2)B(1) in-plane bending mode (ν(12)) at 344.2 (±0.8) cm(-1) in a cryogenic argon matrix. This is the first experimental observation of ν(12) for the propargyl radical. In the previous study, ν(12) had been fixed at 333 (±10) cm(-1) based on detection of its overtone (2ν(12), 667.7 ± 1.0 cm(-1)) and a possible combination band (ν(10) + ν(12), 1339.0 ± 0.8 cm(-1)). The new observation is in reasonable agreement with this previous experimental estimation of ν(12). Ab initio coupled cluster anharmonic force field calculations were used to guide the analysis. Our finding completes the fundamental frequency database for propargyl.