A full-length cDNA of a masquerade-like serine proteinase homologue (PmMasSPH) of Penaeus monodon was cloned and characterized by rapid amplification cDNA end (RACE) method. The complete cDNA sequence of 1958bp contains an open reading frame (ORF) of 1572bp, encoding a 523 amino acid protein including a 19 amino acid signal peptide. The calculated molecular mass of the mature protein (504 amino acids) is 51.58kDa with an estimated pI of 4.86. PmMasSPH has most of the structural characteristics of insect prophenoloxidase activating factors (PPAFs) (the N-terminal clip domain and the C-terminal serine proteinase-like domain) but in the N-terminal region there are extensive glycine-rich repeats (LGGQGGG). Sequence comparison showed that the deduced amino acid of PmMasSPH has an overall similarity of 69%, 68% and 61% to those of Apis mellifera PPAF, Callinectes sapidus PPAF and Tenebrio molitor PPAF, respectively. A neighbour-joining tree revealed a clear differentiation of each species and also indicated that PmMasSPH and C. sapidus PPAF are closely related phylogenetically. In situ hybridisation and real-time RT-PCR analyses showed that PmMasSPH transcript in haemocytes of P. monodon increased within 24h after Vibrio harveyi injection.