Fog data of Southern Germany from the period 1949--1990 indicate a significant urban influence on fog frequency. An increase of the urban building density is connected with a reduction in the average number of fog days, which is interpreted as an effect of the urban heat island and moisture deficit. Feedback mechanisms which intensify the urban-rural contrast are discussed. The results are transferable to large cities with relatively good air quality. The study is a contribution to the controversial discussion about the urban effect on fog frequency. ¿ American Geophysical Union 1995