In 1949, 1973 and 1984 the wood of Staelduinen near Hook of Holland was surveyed for epiphytic lichens. After a sharp decline in the quality of the epiphytic flora from 1949 till 1973, a slight improvement was noticed in 1984, corresponding with a decrease of the concentration of sulphur dioxide in the air (fig. 1). It will probably take many more years for the epiphytic lichen flora to regain the 1949-quality-level.