The high-resolution (0.005 cm-1) Fourier transform infrared spectrum of PH3 is recorded and analyzed in the region of the fundamental stretching bands, v1 and v3. The v2 + v4 and 2v4 bands are taken into account also. Experimental transitions are assigned to the v1, v3, v2 + v4, and 2v4 bands with the maximum value of quantum number J equal to 15, 15, 13, and 15, respectively. a1-a2 splittings are observed and described up to the value of quantum number K equal to 10. The analysis of a1/a2 splittings is fulfilled with a Hamiltonian model which takes into account numerous resonance interactions among all the upper vibrational states.