The viscometric behaviour of high molecular mass polyamide 6, prepared by activated anionic polymerization of epsilon-caprolactam in a twin-screw extruder, has been studied in 90% formic acid. Results obtained show that viscosity number increases with polymer concentrations to approximately 0.001 or 0.001 5 g ml-1, but unexpectedly declines thereafter. Whereas anomalous results are obtained at a concentration level of 0.005 g ml-1, as specified in ISO 307:1984, at very low solution concentration, viscometry data correlate well with findings from g.p.c.