Glycogen synthase kinase‐3 is involved in diverse functions including insulin signalling and development. In a number of substrates, phosphorylation by glycogen synthase kinase‐3 is known to require prior phosphorylation at a Ser in the +4 position relative to its own phosphorylation site. Here we have used synthetic peptides derived from a putative glycogen synthase kinase‐3 site in the Drosophila translation initiation factor eIF2Bϵ to investigate the efficacy of residues other than Ser(P) as priming residues for glycogen synthase kinase‐3β and its Drosophila homologue Shaggy. glycogen synthase kinase‐3β phosphorylated peptides with Ser(P) and Thr(P) in the priming position, but peptides with Tyr(P), Thr, Glu or Asp were not phosphorylated. The V max for the Thr(P) peptide was three times higher than that of the Ser(P) peptide. These data suggest that glycogen synthase kinase‐3 is unique among phosphate‐directed kinases. The priming site specificity of Shaggy is similar to that of mammalian glycogen synthase kinase‐3β. This unpredicted efficacy of Thr(P) in the priming position suggests that there may be other unidentified substrates for these kinases.