Partial and total loss of function mutant alleles of a putative Drosophila homologue (DPhK-gamma) of the vertebrate phosphorylase kinase gamma-subunit gene have been isolated. DPhK-gamma is required in early embryonic processes, such as gastrulation and mesoderm formation; however, defects in these processes are seen only when both the maternal and zygotic components of DPhK-gamma expression are eliminated. Loss of zygotic expression alone does not appear to affect normal embryonic and larval development; some pupal lethality is observed but the majority of mutant animals eclose as adults. Many of these adults show defects in their leg musculature (e.g. missing and degenerating muscles), in addition to exhibiting melanised "tumours" on their leg joints. Loss of only the maternal component has no obvious phenotypic consequences. The DPhK-gamma gene has been cloned and sequenced. It has an open reading frame (ORF) of 1680 bp encoding a 560 amino acid protein. The predicted amino acid sequence of DPhK-gamma has two conserved domains, the catalytic kinase and calmodulin-binding domains, separated by a linker sequence. The amino acid sequence of DPhK-gamma is homologous to that of mammalian PhK-gamma proteins but differs in the length and amino acid composition of its linker sequence. The expression of DPhK-gamma mRNA is developmentally regulated. We discuss the implications of these observations.