There are a number of different glycoproteins that have been identified relatively recently which contain oligosaccharides linked to serine or threonine in a peptide backbone via phosphodiesters. It is possible that these glycoproteins may form an alternative structural class of glycosylation. This modification has been referred to as phosphoglycosylation (Mehta et al., 1996; J. Biol. Chem., 271, 10897-10903), and has been reported in slime molds and several unicellular parasites. In this review, examples of phosphoglycosylation from different biological sources are discussed. Those which are well characterized have been found to be highly variable with respect to the glycan moiety, while sharing some common features. An experimental approach detailing how to determine whether a protein is phosphoglycosylated is also presented.