ABSTRACT: The magnitude of El Niño in 1997-98 was one of the strongest of the century. A series of red tides occurred in Hong Kong territorial waters between mid-March and mid-April 1998, resulting in the loss of HK$250 (US$32) million in fish killdamage. The causative species was Gyrodinium aureolum. We used a time series of satellite images to track the development of the harmful algal bloom and relate its movement to physical oceanographic conditions. Prior to the red tide event inHong Kong, harmful algal blooms had occurred earlier (November 1997-February 1998) along the south China coast to the east of Hong Kong, although the species were different. The progression of harmful algal blooms from northeast to southwest coincidedwith the southwesterly movement of the south China Coastal Current as indicated by the satellite images of SST (sea surface temperature) from the satellite AVHRR. Remote sensing images of chlorophyll a (chl a) from SeaWiFS (sea-viewing widefield of view sensor) confirmed high near-shore chl a for the same region. The entire event coincided with the dramatic change in the oceanographic conditions of the northern portion of the South China Sea between 1997 and 1998 for the period ofMarch to mid-April, as revealed in the weekly composite SST. The SST images showed a warm tongue pointing north to the south China coast in 1998 versus a cold tongue pointing south in 1997 in the middle of the South China Sea. The differences are believedto be due to El Niño and responsible for setting up the physical oceanographic conditions which were favorable for the formation of harmful algal blooms along the south China coast. The warm tongue in SST suggested that the warm water from the SouthChina Sea might have been piling up towards the south China coast. On the other hand, downwelling of the south China Coastal Current along the coast due to the Northeast Monsoon during March might have been moving against the South China Sea warm water atthe bottom. As a result, the coastal waters of the south China coast including Hong Kong became trapped along the coast. Given local eutrophied conditions of the China coast, the outbreak of harmful algal blooms occurred over a coast-wide scale (~400 km)in winter 1997 and spring 1998. It appears that the use of satellite SST images is helpful in detecting large-scale changes in oceanographic conditions and relating the changes to possible outbreak of harmful algal blooms.