The nucleotide and deduced amino acid sequence of a serine protease (AgSp14D1) from the human malaria vector, Anopheles gambiae, is presented. The gene product is a 360 amino acid protein that contains two domains and has the highest sequence similarity to the Drosophila melanogaster serine protease easter and to prophenol oxidase activating enzyme (pPAE) from Manduca sexta. The catalytic domain is at the carboxy terminus and has the conserved serine, histidine and aspartic acid residues found in serine proteases as well as six cysteines common to invertebrate enzymes. The amino terminus contains critical cysteines that define a clip (=disulphide knot) domain which places this gene product in a subfamily of regulatory serine proteases that includes not only easter and pPAE but also the Drosophila proteins masquerade, stubble and snake as well as proclotting enzyme and factor B from the horseshoe crab. In situ hybridization to the polytene chromosomes detects a single band at 14D and Southern analysis with a probe from the 5' end of the gene confirms the single copy status of this gene. Northern analysis reveals changes in transcript abundance during development and following blood feeding. Interestingly, this analysis also shows an increase in transcript levels following wounding or injection of bacteria.