The general eastward flow in the north Indian Ocean during summer, which is called the Southwest Monsoon Current (SMC), flows eastward south of India, turns around Sri Lanka, and enters the Bay of Bengal. The intrusion of the SMC into the Bay of Bengal is studied using expendable bathythermograph (XBT) observations along the shipping route between Sri Lanka and Malaca Strait, TOPEX/POSEIDON sea surface height anomalies, and an ocean general circulation model. The intrusion appears first as a broad northward shallow (confined to the upper 200 m) flow in the central part of the Bay of Bengal during May. As the summer season advances, it moves westward, intensifies, and becomes narrow. The mean seasonal (May--September) transport of the SMC into the Bay of Bengal is about 10 Sv. The zonal variation of the geostrophic velocity across 6 ¿N calculated using the XBT data compares well with that from TOPEX/POSEIDON altimeter data. However, the SMC in the XBT data is faster (40 cm s-1) than in the altimeter data or the numerical simulation (25 cm s-1). Harmonic analysis of the depth of 20 ¿C isotherm together with a simple forced Rossby wave model demonstrates that the SMC east of Sri Lanka is forced by both Ekman pumping in the Bay of Bengal and Rossby wave radiation associated with the spring Wyrtki <1973> jet in the equatorial Indian Ocean. The initial intrusion of the SMC into the Bay of Bengal is attributed to this Rossby wave radiation from the eastern boundary. ¿ 1999 American Geophysical Union