  Observations with the Short Wavelength Spectrometer (SWS) onboard the {\it
Infrared Space Observatory} (ISO) have led to the first detection of the methyl
radical ${\rm CH_3}$ in the interstellar medium. The $\nu_2$ $Q-$branch at 16.5
$\mu$m and the $R$(0) line at 16.0 $\mu$m have been unambiguously detected
toward the Galactic center SgrA$^*$. The analysis of the measured bands gives a
column density of (8.0$\pm$2.4)$\times10^{14}$ cm$^{-2}$ and an excitation
temperature of $(17\pm 2)$ K. Gaseous ${\rm CO}$ at a similarly low excitation
temperature and ${\rm C_2H_2}$ are detected for the same line of sight. Using
constraints on the ${\rm H_2}$ column density obtained from ${\rm C^{18}O}$ and
visual extinction, the inferred ${\rm CH_3}$ abundance is
$(1.3{{+2.2}\atop{-0.7}}) \times 10^{-8}$. The chemically related ${\rm CH_4}$
molecule is not detected, but the pure rotational lines of ${\rm CH}$ are seen
with the Long Wavelength Spectrometer (LWS). The absolute abundances and the
${\rm CH_3/CH_4}$ and ${\rm CH_3/CH}$ 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 ${\rm H}$ with polycyclic aromatic hydrocarbons and solid
aliphatic material are included.
