The plane strain fracture toughness of two ductile polymers, ultra high molecular weight polyethylene (UHMWPE) and acrylonitrile-butadiene-styrene (ABS), was measured by using the essential work of fracture approach. Truly plane strain fracture toughness (omega (Ie)) was measured for ABS at quasi-static and impact rates of loading. For UHMWPE, the measured values were only "near" plane strain values (omega (Ie)*). It was confirmed both omega (Ie)* and omega (Ie) were independent of specimen type but dependent on strain rate. For UHMWPE, there was a negative strain rate effect, i.e., omega (Ie)* decreased with increasing loading rate. At low quasi-static loading rate (nu less than or equal to 10 mm/min), omega (Ie)* was constant at 55 kJ/m(2). It then decreased to 15 kJ/m(2) when the loading rate was increased to 100 mm/min, and remained at that value even up to impact rate of loading (nu = 3.7 m/s). For ABS, a mild positive strain rate effect was observed. omega (Ie) increased from 13 kJ/m(2) at nu = 10 mm/min to 17 kJ/m2 at nu = 3.7 m/s.