Observations with the Short Wavelength Spectrometer (SWS) onboard the Infrared Space Observatory (ISO) have led to the first detection of the methyl radical CH_3 in the interstellar medium. The ν_2 Q-branch at 16.5 μm and the R(0) line at 16.0 μm have been unambiguously detected toward the Galactic center SgrA^*. The analysis of the measured bands gives a column density of (8.0±2.4)×10^14 cm^-2 and an excitation temperature of (17± 2) K. Gaseous CO at a similarly low excitation temperature and

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... _2H_2 are detected for the same line of sight. Using constraints on the H_2 column density obtained from C^18O and visual extinction, the inferred CH_3 abundance is (1.3+2.2-0.7) × 10^-8. The chemically related CH_4 molecule is not detected, but the pure rotational lines of CH are seen with the Long Wavelength Spectrometer (LWS). The absolute abundances and the CH_3/CH_4 and CH_3/CH ratios are inconsistent with published pure gas-phase models of dense clouds. The data require a mix of diffuse and translucent clouds with different densities and extinctions, and/or the development of translucent models in which gas-grain chemistry, freeze-out and reactions of H with polycyclic aromatic hydrocarbons and solid aliphatic material are included.