In this paper the urban heat island (UHI) of the city of Salamanca, Spain is examined. The temporal evolution of the intensity of the UHI for the three-year period between 1996 and 1998 was studied by comparing the temperatures in an urban area and those in a nearby rural area. The results pointed to the existence of a nocturnal UHI, whose highest mean value was recorded in autumn (3.2 degreesC). Using the transect selection method, the spatial distribution of the UHI was analyzed. The lowest nocturnal temperatures were observed in the outskirts of the city, mainly in green areas close to the river. The daytime mean temperatures revealed a negative UHI, the lowest temperatures were recorded in the urban area, where the buildings, which afford shade, prevent the arrival of direct solar radiation. Under conditions of atmospheric stability, the UHI is more intense than under conditions of instability.