A distribution survey of Hyla arborea has been carried out in the western part of Zealand Flanders over a period of six years. Additional data on the eastern part and the neighbouring Belgian area have been collected. The relationship between the maximum number of males calling on one evening/night and the estimate of their population size is a suitable basis to predict the total number of males in other pools during a season. Methodological aspects of the fieldwork are discussed. Great fluctuations in presence and activity of the Tree Frog illustrate the need for longterm surveys. Hyla arborea is declining, and nowadays its distribution is restricted to some localities in the western part of the region. These localities are classified according to the number of males and their importance for conservation.