The gas-phase infrared spectrum of natural (\mathrm{CH}{2} \mathrm{FCl}) has been investigated in the (v{3}) region between 1336 and (1365 \mathrm{~cm}^{-1}), at a resolution of (0.002 \mathrm{~cm}^{-1}), using a tunable diode laser spectrometer. This vibration of (A^{\prime}) symmetry species is expected to give origin to an (a / b)-type hybrid band with a structure approaching that of parallel and perpendicular bands of a prolate symmetric top. The rovibrational analysis led to the identification of more than (1300\left(J \leqslant 41, K_{a} \leqslant 17\right)) and (700\left(J \leqslant 42, K_{a} \leqslant 11\right)) transitions of the (\mathrm{CH}{2} \mathrm{~F}^{33} \mathrm{Cl}) and (\mathrm{CH}{2} \mathrm{~F}^{37} \mathrm{Cl} \nu_{3}) band centered at 1353.3173 and (1353.1962 \mathrm{~cm}^{-1}), respectively. From a simultaneous fit of GSCD of this work, together with literature microwave measurements, a set of ground-state constants up to the quartic coefficients has been obtained using the Watson's (A)-reduction Hamiltonian in the (I^{\prime}) representation. A number of (K_{a}) subbands of the main species were found to be affected by Coriolis-type resonance, and the (v_{6}+v_{9}) vibrational level was identified as the perturber. A least-squares fit of the assigned transitions provided a set of effective upper-state constants for both the isotopomers. From spectral simulation it was found that the (\nu_{3}) band of (\mathrm{CH}_{2} \mathrm{~F}^{35} \mathrm{C}) is composed of about 80 and (20 % a) - and (b)-type components, respectively. (c) 1993 Academic Press, Ins.