  Using the Effelsberg 100-m telescope, absorption in the (J,K) = (1,1), (2,2)
and (3,3) inversion lines of ammonia (NH_3) was detected at a redshift of z =
0.6847 toward the gravitational lens system B0218+357. The lambda ~ 2cm
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
~35K, corresponding to a kinetic temperature of ~55K. The column density toward
the core of image A then becomes N(NH_3) ~ 1 * 10^(14)cm^(-2) and fractional
abundance and gas density are of order X(NH_3)~10^(-8) and n(H_2)~5 *
10^(3)cm^(-3), respectively. Upper limits are reported for the (2,1) and (4,4)
lines of NH_3 and for transitions of the SO, DCN, OCS, SiO, C_3N, H_2CO, SiC_2,
HC_3N, HC_5N, and CH_3OH 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.
