Broad scale horizontal variations of the main thermocline south of Bermuda have been revealed by an extensive ocean measurements program over several years. The primary physical property of the ocean sampled during these studies was the sound speed because of interest in underwater sound propagation. For the oceanographer the sound speed measurements in the main thermocline provide information similar to ocean temperature measurements except that there is also a dependence of the sound speed on the salinity and the pressure. In the summer of 1962, 31 sound speed profiles were measured. This measurement was followed in the summer of 1964 with 67 sound speed profiles. Complex horizontal variations of a surface of constant sound speed found near the depth of 800 m were the beginning indications of spatial scales of about 240¿60 miles (440¿110 km) of undulations in the main thermocline. In the summer of 1966, 115 sound speed profiels were measured, most of which were distributed on a rhombic grid pattern with 65 miles (120 km) between stations. These observations are compared with the 1962 and 1964 measurements. The 1966 measurements confirm the existence of spatial scale of about 240 miles (¿60 miles) (440¿110 km) as well as the existence of other scales in this ocean region. The spatial variations are indicated by a contour pattern of the depth variations (~300 m) of a 1502-m/s sound speed surface near the depth of 800 m in the main themocline, where the vertical thermal gradient is large. The observations are interpreted as evidence for Rossby waves or baroclinic waves possibly of finite amplitude that show northwest-southeast alignment.