To obtain a water-soluble metal working fluid compsn. which is excellent in resistance to rotting, in separation from a lubricating oil getting mixed, and in working performance by incorporating at least one polyalkylene glycol deriv. having a specified number average mol.wt. into the same.

This compsn. contains, as a base oil, at least one polyalkylene glycol deriv. of the formula: R1-O-(PO)a(EO)b(PO)c-H (wherein R1 is a 1-18C alkyl; PO is oxypropylene; EO is oxyethylene; and (a), (b), and (c) are numbers satisfying the mol.wt., each being an integer of 1 or higher). The number average mol.wt. of the deriv. is 500-20,000. Esp. pref., the molar ratio of PO/EO, i.e., (a+c)/b, in the formula is (3/7)-(6/4), and the content of the deriv. in the compsn. is pref. 0.5-50 wt.%.