||The formyl peptide N-formylmethionine-leucine-phenylalanine (fMLP) is a potent chemo-attractant for mammalian neutrophils. In this study, we have tested the hypothesis that fMLP binds to haemocytes of the penaeid prawn Penaeus penicillatus. Fluorescence microscopy and flow cytometric analysis, with rhodamine-fMLP, demonstrated that fMLP binding was present only in sub-populations of the haemocytes, granulocytes and semi-granular cells. In addition, fMLP triggered chemotaxis in sub-populations of haemocytes in a dose-dependent manner at nM range. Microphysiometry experiments demonstrated rapid extracellular acidification upon addition of fMLP. However, the concentrations required to elicit these response decreased in sensitivity from binding, chemotaxis to extracellular acidification. Tert-Butoxy-carbonyl (t-BOC), a specific fMLP receptor antagonist in mammalian cells, was able to block binding, chemotaxis, and extracellular acidification induced by fMLP in prawn haemocytes. Though preliminary data showed that fMLP was not efficient in mediating phagocytosis, the ability of prawn haemocytes to migrate toward fMLP in vitro suggests that this mechanism may be important for the accumulation of these cells in infected tissues. This study suggests the presence of putative fMLP receptors in prawn haemocytes and raises the possibility that the chemotactic peptide functions in the innate immunity of crustaceans. The presence of a putative fMLP receptor on haemocytes and the induced extracellular acidification of haemocytes together imply similarities between the signaling pathways of prawn haemocytes and mammalian neutrophils.