The crystallites in a polycrystalline target are considered to belong to two exclusive subsets. The first subset consists of crystallites with main crystallographic axes within the critical angle for channelling while the second subset consists of crystallites with axes larger than the critical angle for channelling. Single crystal and amorphous sputtering theory is used to calculate the sputtering yield for the different subsets and an average value for polycrystalline targets is determined. This value is shown to differ significantly from the Sigmund value for amorphous targets.