Microtwins in semi-metal thulium phosphide (TmP) epilayers grown on (001) GaAs substrates by molecular beam epitaxy have been studied by transmission electron microscopy. Selected area diffraction patterns show extra spots along [111] and [224] corresponding to three times the normal rock-salt periodicity. Only one or two twin variants are found in the crystal. The occurrence of the observed microtwins in the TmP-GaAs heterostructure can be accounted for by the growth-accident mechanism, i.e., the formation of microtwins is via growth accidents in the stacking sequence on [111] and [112] planes. The growth accidents appear to occur due to rapid growth rates and/or contamination. (C) 2002 Elsevier Science B.V, All rights reserved.