Both experimental and theoretical infrared investigations of cyclopentadienylphosphine (CpP) are reported. The infrared spectra (3500-500 cm(-1)) in the gas phase have been recorded at 0.5 cm-1 resolution. Infrared absorptions bands of the two lowest stable conformers were observed and assigned. Average integrated intensities of isolated and overlapping vibrational bands were also determined experimentally. The vibrational frequencies of the CpP system and its P-dideuterated isotopologue have been calculated by means of density functional theory. The Becke exchange functional and Lee-Yang-Parr correlation functional method with a combination of the two basis sets, namely 6-31 +G(d,p) and the correlation-consistent triple-xi cc-pVTZ set of Dunning, were used. Hybrid B3LYP/B3LYP//cc-pVTZ/6-31+G(d,p) anharmonic frequencies of the fundamental, overtone, and combination transitions were calculated in the 3500-200 cm(-1) area with the use of a variational approach, implemented in the P_Anharv_v1.1 code, to assign the experimental data for each conformer.