Hexokinase has been purified from adult Schistosoma mansoni worms and the activity shown to be associated with a single protein species having an M(r) about 50,000. This protein is recognized on Western blots probed with antisera against rat Type I hexokinase or against a recombinant S. mansoni hexokinase that had been expressed in Escherichia coli using a previously cloned cDNA. An 18-residue N-terminal sequence determined for the purified S. mansoni hexokinase is identical to that deduced from the nucleotide sequence of the cDNA, consistent with the view that the cloned cDNA encodes the hexokinase characterized in the present study. The S. mansoni enzyme has a relatively low Km (approximately 60 microM) for glucose and is sensitive to inhibition (competitive versus ATP, Ki approximately 50 microM) by its product, glucose 6-phosphate (Glc-6-P). With these kinetic properties and 50 kDa molecular mass, S. mansoni hexokinase resembles the ancestral hexokinase predicted to have given rise, by gene duplication and fusion, to the present day 100-kDa Glc-6-P-sensitive mammalian hexokinases. The schistosomal hexokinase represents the first 50-kDa Glc-6-P-sensitive hexokinase whose sequence has been obtained. The schistosomal hexokinase does not bind to mitochondria, consistent with its lack of a hydrophobic segment at the N terminus which is required for binding of the mammalian Type I and II isoenzymes to mitochondria. The marked Crabtree effect exhibited by S. mansoni cercariae may be at least partly attributed to the expression of rather high levels of a hexokinase having a high affinity for glucose but only a moderate sensitivity to product inhibition by Glc-6-P.