Spironucleus is a genus of small, flagellated parasites, many of which can infect a wide range of vertebrates and are a significant problem in aquaculture. Following the ban on the use of metronidazole in food fish due to toxicity problems, no satisfactory chemotherapies for the treatment of spironucleosis are currently available. Using membrane inlet mass spectrometry and automated optical density monitoring of growth, we investigated in vitro the effect of Allium sativum (garlic), a herbal remedy known for its antimicrobial properties, on the growth and metabolism of Spironucleus vortens, a parasite of tropical fish and putative agent of hole-in-the-head disease. The allium-derived thiosulfinate compounds allicin and ajoene, as well as an ajoene-free mixture of thiosulfinates and vinyl-dithiins were also tested. Whole, freeze-dried garlic and allium-derived compounds had an inhibitory effect on gas metabolism, exponential growth rate and final growth yield of S. vortens in Keister's modified, TY-I-S33 culture medium. Of all the allium-derived compounds tested, the ajoene-free mixture of dithiins and thiosulfinates was the most effective with a minimum inhibitory concentration (MIC) of 107 μg ml(-1) and an inhibitory concentration at 50% (IC(50%)) of 58 μg ml(-1). It was followed by ajoene (MIC = 83 μg ml(-1), IC(50%) = 56 μg ml(-1)) and raw garlic (MIC20 mg ml(-1), IC(50%) = 7.9 mg ml(-1)); allicin being significantly less potent with an MIC and IC(50%) above 160 μg ml(-1). All these concentrations are much higher than those reported to be required for the inhibition of most bacteria, protozoa and fungi previously investigated, indicating an unusual level of tolerance for allium-derived products in S. vortens. However, chemically synthesized derivatives of garlic constituents might prove a useful avenue for future research.