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K-band radio observations of comet Hale–Bopp (C/1995 O1)) were conducted in March/April 1997 at the 100-m Telescope of the Max-Planck-Institut für Radioastronomie. Emission was firmly detected from the five lowest metastable (J = K)) inversion transitions of ammonia. Assuming a thermal distribution for the metastable states of NH3, we derive a rotational temperature of 104 ± 30 K and an ammonia production rate at perihelion of 6.6 ± 1.3 × 1028s-1. The updated ammonia-to-water abundance ratio is found to be of the order of 1.0%. We also report a marginal detection of the 616–523transition line of water at λ = 1.35 cm.