The aim of the study was to investigate the formation intensity of the urban heat island (UHI) in Białystok, a relatively small city in north-eastern Poland. The UHI development was determined on the basis of meteorological data obtained from two weather stations of the Regional Inspectorate for Environmental Protection in Białystok, one located in the city center, and the other on its outskirts. The measurements of temperature gathered by automobile traverses along streets running from the city limits through the center were carried out in order to provide spatial diversity of the studied phenomenon. The results indicated that UHI in Bialystok is very clear. The intensity of this phenomenon changes in the daily cycle, with the peak in the evening. During the year the heat island is the most pronounced in the spring.