Previous french studies have placed Nectria coccinea’s part in a prominent position in mycoflora associated with beech bark necrosis after Cryptococcus fagisuga pullulations. Artificial inoculations have demonstrated N. coccinea ability to produce restricted beech bark decay. Necrosis’s perennity and spread seem to be conditionned by C. fagisuga population’s density. Results of 1976-1977 experimental studies, lay down necessary association of two organisms implicated : Each day, from October 1976 to June 1977, germinating ascospores have been sprayed on beech bark infested with beech scale (population density 3-4 E. J. Parker, scoring system). Two kinds of symptoms, red bark discoloration and slime flux, reveal successful N. coccinea infection, and corresponding necrosis (reisolation, november 1977). These infections occur especially in November and May-June. Although, these results prove that C. fagisuga assists N. coccinea beech bark enter, infection courts and processes remain unprecises. Artificial bringing of N. coccinea (mycelium, needle wounded) in beech bark. leed to a necrosis, spreading of wich depend on C. fagisuga density population. Restricted to one or two centimeters square, in lack of beech scale, necrosis rapidly extend to several decimeters square, where C. fagisuga are abounding. C. fagisuga brings a bark physiological disorder, rendering bark more vulnerable to N. coccinea destructive ability. Mechanical (vaccum cleaner, brush) or chemical (oleoparathion) elimination of beech scale on N. coccinea infected (or no) bark, confirms previous observations. Infections never occur, in lack of beech scale, or after insecticide treatment. Spread of beech bark necrosis stops, and a beginning of healing over occurs where C. fagisuga has been eliminated. Unlike, on same trees, necrosis extend in C. fagisuga infested place. It seems that beech scale disturbs natural physiological control processes. In conclusion, C. fagisuga assists, in peculiar manner N. coccinea implantation in bark of beech, brings a bark physiological disturbance, leading easier N. coccinea decay ability. Beech scale, necessary precursor of N. coccinea, take a prominent part in beech bark disease. Stands where disease, and particulary beech scale, are endemic, would be searched after, independently of climatic fluctuations. This imply a careful sanitary gestion and perhaps proscribes pure beech stands, always restored in the same place. Variations in some beech susceptibility to beech scale and to N. coccinea, as site situations influences on disease severity, would reveal advantageous in beech bark disease control method.