Phonons in deuterium cyanide have been measured by neutron coherent inelastic scattering. The main subject of study was the transverse acoustic mode in the (110) direction polarised along (110) which is associated with the first-order structural phase transition at 160K. Measurements have shown that the frequency decreases by about 25% between about 225 and 160K as the transition temperature is approached. The other acoustic modes observable in the a*b* scattering plane have been measured and show no anomalous temperature dependence. Optic modes were unobservable because of the small size of the single-crystal sample which gave insufficient scattered intensity. Apart from the 'soft' mode, the measured frequencies are in good agreement with lattice dynamics calculations.