Abstract: A process for preparing highly viscous polyhexamethyleneadipamide by postpolycondensing the melt of a polymer having a relative viscosity of 2.5-2.7 (measured in a 1% strength solution in 96% strength sulfuric acid) in a twin-screw extruder having degassing zones and closely meshing screws which turn in the same direction. The polymer melt is polycondensed in the extruder by treatment in a plurality of serially arranged zones of alternatingly elevated and reduced pressure, wherein in the zones of elevated pressure, steam superheated to the temperature of the melt is supplied to the melt in an amount of 0.05-0.7 kg/kg of polymer, whereas in the zones of reduced pressure, the input vapor phase and water of condensation are drawn off. The temperature of the melt is raised at a uniform rate over the length of the extruder from 260.degree.-280.degree. C. in the first zone to 280.degree.-310.degree. C. in the last zone.