New measurements of the velocity of sound in water over a wide range of temperatures and pressures have been made and are used to calculate the nonlinearity parameterB/Ain the temperature range from 0° to 80°C and at pressures to about 10 000 kg/cm2. It is found that, for low temperatures,B/Arises to a broad maximum at about 3000 kg/cm2and then slowly decreases. At the highest temperatures,B/Adecreases very slowly with increasing pressure. At 1 atm pressure, theB/Avalue for 0°C is the lowest calculated at any temperature and pressure, andB/A increasesmonotonically with temperature in the range covered. However, at 3000 kg/cm2, theB/Avalue for 0°C is the largest obtained, andB/A decreaseswith increasing temperature. The values ofB/Arange from a low value of 4.08 at 1 atm and 0°C to 6.80 at 3000 kg/cm2and 0°C.B/Ais found to be relatively insensitive to temperature and pressure. The usefulness ofB/Ameasurements as a means of studying the liquid state is discussed.