An antifungal peptide, PvHCt, which corresponds to the 23 amino acid C-terminal sequence of the shrimp respiratory protein hemocyanin, has been previously identified in the plasma of the penaeid shrimp Litopenaeus vannamei. It is generated by proteolytic cleavage in response to a microbial challenge. Similarly, a C-terminal fragment of hemocyanin displaying antimicrobial activity has been isolated from crayfish plasma. These peptides are believed to contribute to the crustacean defence. The phenomenon of in vitro antimicrobial peptide generation from a respiratory pigment already observed with hemoglobin thus appears not to be restricted to mammals. PvHCt displays a broad spectrum of antifungal activity with minimum inhibitory concentrations (MICs) in the range 3-50 µM (12.5 µM against the shrimp pathogen Fusarium oxysporum). Its activity would be based on the inhibition of spore germination. To contribute to the elucidation of the mechanism of PvHCt antifungal activity, we determined its three-dimensional structure by circular dichroism (CD), NMR and molecular modelling and examined its effects on the F. oxysporum spore ultrastructure by transmission electron microscopy (TEM). CD and NMR data indicate that PvHCt is unfolded in an aqueous environment but adopts a similar helical structure in methanol solution and in dodecylphosphocholine (DPC) micelles used to mimic biological membranes. The structure consists of an amphipathic α-helix spanning residues 8 to 18. TEM shows that PvHCt induces structural changes of the plasma membrane accompanied by a disorganization of the cytoplasm and a significant decrease of lipid bodies.