Raman scattering and infrared reflectivity data are presented for AgGaS2, and they provide fairly complete information on the frequencies of the zone-centre lattice vibrations. Like many ternary semiconductors AgGaS2 has the chalcopyrite structure, and this is treated as a perturbation of the zincblende structure found in binaries such as GaP and ZnS. The probable form of the zine-centre modes in the ternary compounds is studied in detail, partly by considering the form of corresponding modes in the binaries, and partly by drawing a simple analogy with a tetra-atomic linear chain. This leads to the conclusion that in the high-frequency modes of the ternaries both cations are vibrating, in contradiction with previous work. Approximate atomic displacements have been derived for all the zone-centre modes.