An experimental ultrasonic study has been made of the elastic and nonlinear acoustic behaviour of single-crystal triglycine sulphate (TGS) in both the ferroelectric and paraelectric phases. The ultrasonic wave velocities for 12 modes of vibration propagated along the [100], [010], [001] and [101] directions have been measured as functions of temperature in the range 293± 338 K, which covers the Curie temperature T C , and hydrostatic pressure up to 0.2 GPa at selected temperatures in the same temperature range. The results provide ten out of the 13 elastic sti ness tensor components C ij of TGS (the exceptions being C 12 , C 23 and C 25 ) and their hydrostatic-pressure derivatives as a function of temperature. Marked anomalies are observed in the C ij …T † in the vicinity of