A detailed hydrological survey of the coastal waters of Hong Kong has been undertaken over a period of 12 months from 1972-1973. Five sites transecting Hong Kong from north-west to south-east and anticipated as representing typically oceanic and very exposed conditions in the southeast and a sheltered, estuarine environment in the northwest with stations of intermediate hydrology between were selected for study. A number of important hydrological parameters (i.e. actual water temperature, monthly minimum and maximum water temperatures, salinity, chlorinity, conductivity, pH, dissolved oxygen, biological oxygen demand, inorganic phosphate, nitratenitrite, reactive silicate and calcium) have been taken into account. These hydrological parameters were measured three times each month from the surface and bottom waters of the five sites. Two additional surveys were carried out at the most northwesterly site, in winter and summer, respectively, in which the variations in salinity, conductivity, water temperature, and pH of the surface waters were recorded hourly for 24 hours. The spatial and the seasonal variations in the levels of these hydrological parameters have been discussed and further related to local pollution levels as well as to the seasonal influx of fresh water from the Pearl river. These two factors are, it is considered, the most important elements influencing the coastal hydrology of Hong Kong and thus the littoral ecology. Copyright © 1975 Published by Elsevier Inc.