A putative protein kinase gene (PfPK2) has been isolated from the human parasite Plasmodium falciparum by using a mixed oligonucleotide pool which corresponds to a highly conserved region of serine/threonine protein kinases. The complete nucleotide sequence of 5 kb suggests the existence of a second transcriptional unit besides that of the PfPK2 gene, separated by a highly (A+T)-rich region and transcribed in a different orientation. No intron sequence exists in PfPK2. The predicted amino acid sequence of PfPK2 contains features characteristic of eukaryotic serine/threonine protein kinases. Within its putative catalytic domain it shares 33%, 30%, and 28% amino acid identities with rat calcium-calmodulin-dependent protein kinase, human protein kinase C, and bovine cAMP-dependent protein kinase, respectively. Outside the catalytic domain, however, PfPK2 has no homology with regulatory domains of other protein kinases, indicating PfPK2 might be modulated by signals different from those of higher eukaryotes or might be associated with other regulatory subunits. Using a specific antiserum raised in rabbits against a recombinant fragment of the protein expressed in Escherichia coli, PfPK2 was found to be expressed in a stage-specific fashion and mainly localized in the parasitic membrane.