Magnetic resonance of two types of shallow acceptor centres in CdS has been detected optically by monitoring the intensity of the donor-acceptor green edge emission at 2K. The acceptor resonance spectra can be described by an effective spin S'=1/2 and the values of mod g'/sub /// mod were found to be 2.829+or-0.007 and 2.733+or-0.007. From the positions of the zero-phonon emission peaks, the two corresponding acceptor levels were found to be separated in energy by 7+or-3 meV and are attributed to lithium and sodium respectively. The associated shallow donor resonance was found to have mod g/sub ///D mod =1.7877+or-0.0005 and mod gperpendicular to D mod =1.7720+or-0.0007.