Imposex has been observed in several regions of the world to affect many species of prosobranch gastropods such as Stramonita haemastoma (Linnaeus, 1767). This irreversible sexual anomaly is caused mainly by the accumulation of organotin compounds in tissues. The present study aimed to confirm the existence of imposex and to determine the different levels of infection in individuals of Stramonita haemastoma collected from different stations along the rocky coastline of the Gulf of Tunis. To do this, 10 stations covering the entire rocky coastline of the gulf were sampled during March–May 2012. Rates of occurrence of imposex suggest that sexual modifications occur to different degrees in this species in all sampled stations of the gulf. The most contaminated sites were harbour areas (Sidi Bou Said and La Goulette). In addition, an inverse relationship between the distances from potential sources of pollution and the occurrence of this sexual anomaly was observed. It should be noted that toxic effects induced by marine xenobiotics could cause the loss of biodiversity and the disruption of this ecosystem stability.