Monocrystalline Zn0.62Cu0.19In0.19S (ZCIS) films have been grown epitaxially on (0 0 1) GaP substrates by pulsed laser deposition (PLD). Twins are the dominant defects. However, there is a striking in-plane asymmetry in twin distribution, meaning the nucleation of twins is preferred on the (1 1 1) and ((1) over bar (1) over bar 1) planes of the [1 (1) over bar 0] zone. Their formation is suppressed on the ((1) over bar 1 1) and (1 (1) over bar 1) of the [1 1 0] zone. In addition to the twins perfect 60 degrees dislocations lie in the interface to accommodate the misfit strain. Moreover, a Cu-3 P phase has been formed within the GaP substrates due to Cu out-diffusion from the ZCIS film into the (0 0 1) GaP substrate.