6-Pentafluorophenyltriazolopyrimidines of the formula I in which the substituents are as defined below: R1 is alkyl, haloalkyl, cycloalkyl, halocycloalkyl, alkenyl, haloalkenyl, cycloalkenyl, halocycloalkenyl, alkinyl, haloalkinyl or phenyl, naphthyl, or a five- or six-membered saturated, partially unsaturated or aromatic heterocycle which contains one to four heteroatoms from the group consisting of O, N and S, R2 is hydrogen or one of the groups mentioned under R1, R1 and R2 together with the nitrogen atom to which they are attached may also form a five- or six-membered heterocyclyl or heteroaryl which is attached via N and may contain one to three further heteroatoms from the group consisting of O, N and S as ring members; R1 and/or R2 may be substituted as defined in the description; X is cyano, alkyl, alkoxy, alkenyloxy, haloalkoxy or haloalkenyloxy, processes for preparing these compounds, compositions comprising them and their use for controlling phytopathogenic harmful fungi.