The analysis of the Fourier transform spectrum, with a resolution of 0·008 cm-1, of the combination bands ν2+ ν3(A1species) and ν3+ ν5(Especies) of CH3F has allowed the study ofx-yCoriolis resonance between the states υ2= 1 and υ5= 1. The vibrational levels υ2= υ3= 1 and υ3= υ5= 1 are 15 cm-1apart and the interaction is very strong. The combination of the Coriolis resonance and the essential interaction ofl-type (2, 2) in ν3+ ν5gives rise to a ‘giant’l-type doubling : forJ=25 the shift between the levels is about 17 cm-1.