We report transmission electron microscopy studies of native extended defects in pseudomorphic ZnSe/GaAs (001) and lattice-matched ZnSe-In0.04Ga0.96As (001) heterostructures. The dominant defects present in the layers were identified as Shockley stacking fault pairs lying on (111) and <((11)over bar 1)> fault planes and single Frank stacking faults lying on <((1)over bar 11)> or <(1(1)over bar 1)> fault planes by comparing experimental images with the predictions obtained with the g.b = 0 rule as well as with simulated images.