The invention relates to .alpha.-sulfonylamino-acetonitrile derivatives of the general formula (I) including the optical isomers thereof and mixtures of such isomers, wherein Ar.sub.1, and Ar.sub.2 independently of each other stand for an optionally substituted aryl or heteroaryl group, R.sub.1, and R.sub.2 stand independently of each other for hydrogen, optionally substituted C.sub.1,-C.sub.5alkyl, optionally substituted C.sub.2-C.sub.5alkenyl, C.sub.2-C.sub.5alkynyl or optionally substituted C.sub.3-C.sub.6cycloalkyl; R.sub.3 designates hydrogen, C.sub.3-C.sub.5alkenyl, C.sub.3-C.sub.5alkynyl or optionally substituted C.sub.1,-C.sub.5alkyl; R.sub.4 is optionally substituted C.sub.1,-C.sub.5alkyl, optionally substituted C.sub.2-C.sub.5alkenyl, C.sub.2-C.sub.5alkynyl or optionally substituted C.sub.3-C.sub.6cycloalkyl; R.sub.5 and R.sub.6 are independently of each other hydrogen or optionally substituted C.sub.1-C.sub.5alkyl, optionally substituted C.sub.2-C.sub.5alkenyl, C.sub.2-C.sub.5alkynyl or optionally substituted C.sub.3-C.sub.6cycloalkyl; R.sub.7, and R.sub.8 are independently of each other hydrogen or optionally substituted C.sub.1,-C.sub.5alkyl, optionally substituted C.sub.2-C.sub.5alkenyl, C.sub.2-C.sub.5alkynyl or optionally substituted C.sub.3-C.sub.6cycloalkyl; W designates a bridge selected from --O-- --S(O)m-- or --NR.sub.3--; X designates a direct bond or a bridge selected from --O--, --S(O)m-- or --NR.sub.3--; a and b independently of each other stand for a number 1, 2 or 3; and c and m independently of each other stand for a number zero, 1 or 2. These compounds possess useful plant protecting properties and may advantageously be employed in agricultural practice for controlling or preventing the infestation of plants by phytopathogenic microorganisms, especially fungi.