Forest rangers have been observing symptoms of visible decline in European forests since the early 1970s. Conifers were the first victims but, more recently, hardwoods have been presenting symptoms of damage too. This is especially so for indigenous oaks (Quercus robur L. and Q. petraea (Matt.) Lieblin). Decline of oak forests has been reported periodically in Europe since the early eighteenth century. The present situation of decline started in the 1980s. In Belgium, several thousands of oaks have died since 1985 and thousands more are seriously threatened with initially associated with dieback. This sudden decrease in vitality, associated initially with air pollution and acid rain, results from more complex causes in which, as a result of a weakening of natural tree defense mechanisms, several fungi could be involved. Ceratocystis piceae (Münch) Bakshi was isolated from declining oaks in the « Forêt de Soignes » (Belgium), by means of a particular trapping technique. Wood samples from declining oaks to be tested were placed in a liquid medium together with twigs of healthy oaks previously uncorked and sterilized. Air bubbling in the system favours the development of Ceratocystis piceae, despite the presence of secondary microorganisms which are very competitive on agar gel. The presence of Ceratocystis piceae was revealed after 1 - 2 weeks by the formation of synnemata and perithecia on the emerging part of the twigs. The simplicity of the trapping technique used, since it does not require conditions of perfect sterility, makes it a useful isolation method for practical purposes. Its selectivity was also a positive feature : the growth conditions enabled C. piceae to overcome competition from the secondary agents inevitably, necessarily present in tested samples. It should be pointed out, however, that the presence of Fusarium solani (Mart.) Sacc., associated with C. piceae, was recorded in several isolates of declining oak sapwood. A few years ago the distribution of oak Ceratocystis appeared to be limited ...