1. Discharged effluent waters from directly cooled power stations in temperate zones are some 8-12°C warmer than intake temperatures.2. The direct effects of thermal discharges on marine organisms fall into four categories, the mean temperature in relation to normal , the absolute temperature (as it may approach lethal levels), short-term fluctuations in temperature and potential barriers to fish migration. These conditions tend to favour eurythermal species, notably those from the littoral or warmer zoogeography, while inhibiting stenotherms or cooler-water species.3. A potential increase in background temperature would exacerbate the effects of mean and absolute temperature, although operating on a previously selected local community. While a natural temperature rise of 1°C above normal can have a marked impact on even littoral species, any effluent temperature effects will be localized within a small area.