<i>Background:</i><i>Ascaris lumbricoides</i> induces a Th2 response and specific IgE synthesis in humans. This confers antiparasite immunity but could modify the natural history of allergic diseases in the tropics, justifying the study of its allergenic composition. We analyzed the allergenic properties of <i>Ascaris</i> tropomyosin and the frequency of sensitization in subjects exposed to the parasite. <i>Methods:</i> cDNA was obtained by reverse transcription PCR, cloned into pQE30-UA and purified as a 6× His-tagged protein. Equivalence with its natural counterpart was analyzed by cross-inhibition and liquid chromatography-tandem mass spectrometry. Specific IgE was measured by ELISA in 175 asthmatics and 170 nonasthmatics naturally exposed to the parasite and sensitized to the <i>Ascaris</i> extract. <i>Results:</i> The cDNA encoded 287 residues with high sequence identity with other invertebrate tropomyosins. The 40-kDa protein was recognized by human serum and affinity-purified anti-rBlo t 10 IgE. Specific IgE to tropomyosin could represent approximately 50% of the total IgE response to the extract. <i>Ascaris</i> tropomyosin induced wheal and flare in skin prick tests and histamine release from basophils. Although the prevalence of IgE to <i>Ascaris</i> tropomyosin was higher in asthmatic patients, logistic regression analysis suggested that this result was biased by sensitization to mites. <i>Conclusions:</i><i>A. lumbricoides</i> tropomyosin (Asc l 3) is a new allergen that binds specific IgE, induces mediator release from effector cells and is cross-reactive to mite tropomyosins. IgE reactivity to this allergen is very frequent in both asthmatic and normal subjects sensitized to <i>Ascaris</i> extract. The potential role of <i>Ascaris</i> tropomyosin in asthma pathogenesis in tropical regions should be further investigated.