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Using the Effelsberg 100-m telescope, absorption in the ($J,K) = (1,1)$, (2,2) and (3,3) inversion lines of ammonia (NH3) was detected at a redshift of $z = 0.6847$toward the gravitational lens system B0218+357. The $\lambda \sim2$cm absorption peaks at 0.5–1.0% of the continuum level and appears to cover a smaller fraction of the radio continuum background than lines at millimeter wavelengths. Measured intensities are consistent with a rotation temperature of ~35 K, corresponding to a kinetic temperature of ~55 K. The column density toward the core of image A then becomes N(NH$_3) \sim 1\times10^{14}$cm-2and fractional abundance and gas density are of order X(NH$_3) \sim$10-8and n(H$_2) \sim 5\times10^{3}$cm-3, respectively. Upper limits are reported for the (2,1) and (4,4) lines of NH3and for transitions of the SO, DCN, OCS, SiO, C3N, H2CO, SiC2, HC3N, HC5N, and CH3OH molecules. These limits and the kinetic temperature indicate that the absorption lines are not arising from a cold dark cloud but from a warm, diffuse, predominantly molecular medium. The physical parameters of the absorbing molecular complex, seen at a projected distance of ~2 kpc to the center of the lensing galaxy, are quite peculiar when compared with the properties of clouds in the Galaxy or in nearby extragalactic systems.