O.Z. 0050/41784 Oxalic acid derivatives of the formula I A-CO-COO-Y-X-D I where A is an unsubstituted or substituted azetidine ring which is bonded via its nitrogen atom, Y is unsubstituted or substituted alkylene, X is oxygen or sulfur and D is unsubstituted or substituted alkyl or alkenyl or is monocyclic or polycyclic unsubstituted or substituted cycloalkyl, cycloalkylmethyl, cycloalkenyl or cycloalkenylmethyl or unsubstituted or substituted phenyl, phenylalkyl or phenylalkenyl, their preparation and growth regulators containing them.