O.Z. 0050/41753 Oxalylhydroxamic acid derivatives of the formula I R1-ON(R2)-Co-Co-X-R3 I, where the substituents have the following meanings: X is oxygen or sulfur; R1 is substituted or unsubstituted alkyl, alkenyl or alkynyl; substituted or unsubstituted monocyclic or polycyclic cycloalkyl or cycloalkylmethyl; substituted or unsubstituted phenyl or phenyl-C1-C4-alkyl; R2 is hydrogen or alkyl; R3 is substituted or unsubstituted alkyl, alkenyl or alkynyl; substituted or unsubstituted monocyclic or polycyclic cycloalkyl; substituted or unsubstituted phenyl-C1-C4-alkyl or phenoxy-C1-C4-alkyl; and their agriculturally useful salts; their manufacture; growth-regulat-ing agents containing them; and methods of regulating plant growth.