Background: Garlic has a wide range of actions, including antibacterial, antiviral, antifungal, antiprotozoal and antihelminthic actions. This antiparasitic activity has been attributed to allicin, which is the main constituent of garlic. The aim: is to evaluate the potential therapeutic and/or prophylactic effects of allicin on S. mansoni. Method: Swiss albino mice strain CD1were infected with S. mansoni cercariae and sacrificed 40 days later to acquire the adult worms. These worms were collected by perfusion and placed in RPMI medium 1640 at 37°C before transferring to RPMI media containing 0 (control), 5, 10 and 100µg/ml of allicin, where they were incubated for 48h and monitored during this time to evaluate motility and mortality rate by means of stereomicroscope. Twenty male Swiss albino mice strain CD1were divided into two groups. One group was infected with S. mansoni cercariae and treated with allicin one week post infection. The second group was (control). Results: In vitro incubation of S. mansoni adult worms with allicin, showed a statistically high significant difference in comparison with control non-treated group. All worms showed slow movement after 48 h of incubation at concentration of 100µg/ml. No effect was noticed at allicin concentrations of 10 and 5 µg/ml at the end of the experiment. Administration of allicin to the infected mice had non- significant effect on the worm burden. Conclusion: allicin was effective against adult S. mansoni worms in vitro, but with no significant effect in vivo.