We have developed an acoustic thermometer capable of rapidly sensing changes in the spatially averaged temperature over long distances in the ocean, i.e., at basin scales of 1,000 to 4,000 km (Fig. 1). This thermometer is based on measurements of the travel times of acoustic pulses that travel through the interior of the ocean. In one experiment, we were able to recognize changes in the spatially averaged temperature exceeding ±0.02°C in the upper kilometer (Fig. 2). In another experiment, we were able to sense spatially averaged changes of temperature from month to month in the upper 100 m (Fig. 3A) that were not detected with available XBT (expendable bathythermograph), AXBT (airborne XBT), or CTD (Conductivity, Temperature, Depth), observations (Fig. 3B). These are the first acoustic measurements of travel time which have been shown to measure temperature at basin scales (Spiesberger and Metzger, 1991 a; Spiesberger el al., 1992).