Dynamic fatigue crack growth has been investigated in polyethylenes of various molecular weights and branch concentrations. Square load waveforms were applied to sharply notched samples and the load was adjusted so that for all tests the maximum stress intensity factor was constant and tensile, K-max, but different experiments featured different values of minimum stress intensity factor K-min, which was either zero or compressive. The crack growth rate increased as K-min became more compressive. Subsidiary experiments were made using static loads and these confirmed that, as previously reported, the slow crack growth rate decreased with increasing branch concentration and molecular weight. In all the materials studied, fatigue crack growth rates were significantly greater than those under steady loads. However, materials possessing higher branch concentrations showed a relatively greater increase in crack growth rates. For negative values of K-min, increasing branch concentration has a deleterious effect on the fatigue crack resistance. In the fatigue loading experiments studied here it was found that for K-min/K-max approximate to -1, the ranking of materials in terms of crack growth resistance was reversed with respect to that seen under steady loads.