To obtain an alkylhydrazine salt on an industrial scale in a high yield and in a high purity by allowing hydrazine to react with an alkene and water in the presence of a strong protonic acid.

This method for producing an alkylhydrazine salt of formula II (X is the anion of a strong protonic acid) comprises vigorously stirring hydrazine, a protonic acid and an alkene of formula I (R1 is a 1-4C alkyl; R2 is a 1-4C alkyl, a 2-4C alkenyl, a 2-4C alkynyl or phenyl; R3 and R4 are each H, a 1-4C alkyl, a 2-4C alkenyl or a 2-4C alkynyl, or a pair of R1 and R2 or a pair of R3 and R4 each form a 5-8C cycloalkyl together with C) in a reactor at 75-150°C and subsequently cooling the reaction product up to a temperature of ≤60°C. Water serves as a solvent and is added in the form of hyrazine hydrate, an aqueous protonic acid and/or water itself. The hydrazine and the protonic acid are again added to the mother solution, and the alkene is again added to recycle the materials. The alkylhydrazine is useful as an intermediate for medicines or crop-protecting agents and so on.