The rate coefficient has been measured under pseudo-first-order conditions for the Cl + CH3 association reaction at T = 202, 250, and 298 K and P = 0.3-2.0 Torr helium using the technique of discharge-flow mass spectrometry with low-energy (12-eV) electron-impact ionization and collision-free sampling. Cl and CH3 were generated rapidly and simultaneously by reaction of F with HCl and CH4, respectively. Fluorine atoms were produced by microwave discharge in an approximately 1% mixture of F-2 in He. The decay of CH3 was monitored under pseudo-first-order conditions with the Cl-atom concentration in large excess over the CH3 concentration ([Cl](0)/[CH3](0) = 9-67). Small corrections were made for both axial and radial diffusion and minor secondary chemistry. The rate coefficient was found to be in the falloff regime over the range of pressures studied. For example, at T = 202 K, the rate coefficient increases from 8.4 x 10(-12) at P = 0.30 Torr He to 1.8 x 10(-11) at P = 2.00 Torr He, both in units of cm(3) molecule(-1) s(-1). A combination of ab initio quantum chemistry, variational transition-state theory, and master-equation simulations was employed in developing a theoretical model for the temperature and pressure dependence of the rate coefficient. Reasonable empirical representations of energy transfer and of the effect of spin-orbit interactions yield a temperature- and pressure-dependent rate coefficient that is in excellent agreement with the present experimental results. The high-pressure limiting rate coefficient from the RRKM calculations is k(2) = 6.0 x 10(-11) cm(3) molecule(-1) s(-1), independent of temperature in the range from 200 to 300 K.