Wall-pore sensilla in the capsule of Haller's organ on foreleg tarsi of the tick, Amblyomma variegatum, show multicellular responses upon stimulation with human and bovine breath. Filtering breath through charcoal removes the stimulant for some of these receptors. Analysis by gas chromatography coupled with olfactory sensillum electrophysiological recordings indicates that an ethanol extract of the breath components trapped on charcoal contains a major stimulant eluting at the same retention time as H2S. Two types of H2S-sensitive receptors have been identified. They are housed in separate sensilla, and are called sulfide-receptor 1 and 2. Although, both receptor types are characterized by a high sensitivity to H2S with an estimated threshold of ca. 0.1 ppb and a response range covering 5-6 log orders of magnitude, their overall response to sulfides and mercaptans is nevertheless dissimilar. The type 1 receptor fires slightly more upon stimulations with H2S than type 2, whereas ethylmercaptan induces a stronger response from type 2, and dimethyl sulfide activates only receptor 2. In a bioassay, H2S tested at concentrations of ca. 0.02 ppm and 1 ppm equally arouses 60% of resting ticks. Two-thirds of these ticks quest the air with their first pair of legs, and the remainder start active search. By contrast, H2S at ca. 1 ppm in a mixture with CO2 severely diminishes the locomotor stimulating effect of CO2.