The vibrational and K-type rotational levels of the ã, 1A1 and the b, 1B1 states of CH2 have been modelled using the adiabatic approach to the calculation of the Renner–Teller effect, and potential-energy curves derived. Although the overall vibronic pattern is due both to the large amplitude vibration and to the Renner–Teller coupling between the two states which correlate with a 1Δ state of the linear system, some large localised resonances are due to a mixture of Renner–Teller and Fermi resonance coupling.