The present invention relates to an aqueous co-formulation of metalaxyl, which contains: i. metalaxyl, in particular metalaxyl-M； ii. at least one organic pesticide compound PC1, which has a solubility in water of at most 1 g/l at 20°C and a melting point in the range from 40 to 100°C； and iii. an aqueous phase containing water and at least one surfactant； wherein the at least one organic pesticide compound PC1 is present in the form of particles suspended in the aqueous phase and where metalaxyl is essentially present dissolved in the aqueous phase and where the surfactant comprises at least one salt of an oligomer or polymer having a plurality of arylsulfonyl groups. The invention also relates to a process for preparing such formulations, which comprises the following steps: a) providing a suspension of metalaxyl in an aqueous phase containing water, at least one surfactant, which comprises at least one salt of an oligomer or polymer having a plurality of arylsulfonyl groups, b) treating the suspension until metalaxyl is essentially dissolved, to obtain an aqueous composition of metalaxyl, wherein metalaxyl is present in dissolved form； c) mixing the aqueous composition of metalaxyl obtained in step b) with an aqueous suspension of the further organic pesticide compound PC1.
