MnS has been grown by molecular beam epitaxy in the metastable zinc blende (ZB) crystal structure on GaAs (100) substrates using ZnS as a sulphur source. MnS layers were grown over a wide range of growth conditions by varying the growth temperature from 220 degrees C to 300 degrees C and ZnS:Mn flux ratio from 5 to 20. Double-crystal X-ray rocking curves were used to determine the crystallinity and residual zinc content in the layers. This allowed a phase diagram to be produced for ZB MnS giving zinc incorporation as function of the growth temperature and the ZnS: Mn beam flux ratio. In turn, this allowed an optimum growth region to be identified at a growth temperature 240 degrees C and ZnS: Mn flux ratio 6 where ZB MnS with less than 1% residual zinc incorporation were obtained. Under these optimum growth conditions the maximum ZB MnS layer thickness obtained was double that obtained in previous studies, with layers up to 250 nm thick produced.