Tachyplesin 1 is an antimicrobial peptide extracted from hemocytes of the Japanese horseshoe crab Tachypleus tridentatus. We studied the in vitro activity of tachyplesin I against bivalve pathogens: the oyster parasites Bonamia ostreae, the intrahemocytic parasite of the flat oyster Ostrea edulis and Perkinsus marinus, the histozoic parasite of the Eastern oyster Crassostrea virginica, and the bacterium Vibrio P1, pathogenic for the clam Tapes philippinarum. Viability of the protozoans was assessed microscopically by the uptake of the vital dyes acridine orange and ethidium bromide. Following exposure to tachyplesin I, B. ostreae and P. marinus viabilities were reduced in a dose-dependent manner, up to, respectively, 94 and 62% within a 500 μg/ml peptide concentration. The fine structure of P. marinus was highly altered by the peptide. Tachyplesin I also displayed a potent activity against marine vibrios, with a MIC of 0.4-0.8 μg/ml against Vibrio P1. We examined the morphology of oyster hemocytes treated by tachyplesin I, together with the cell functional capabilities to produce chemiluminescence. No effect of the peptide was found on bivalve host cells. As transgenic technology is currently being applied to marine invertebrates, these results indicate that tachyplesin I may provide effective gene sequences to be manipulated in order to produce disease-resistant bivalves.