The inhibitory effect of Lippia alba and Lippia citriodora essential oils on dengue virus serotypes replication in vitro was investigated. The cytotoxicity (CC 50 ) was evaluated by the MTT assay and the mode of viral inhibitory effect was investigated with a plaque reduction assay. The virus was treated with the essential oil for 2 h at 37 o C before cell adsorption and experiments were conducted to evaluate inhibition of untreated-virus replication in the presence of oil. Antiviral activity was defined as the concentration of essential oil that caused 50% reduction of the virus plaque number (IC 50 ). L. alba oil resulted in less cytotoxicity than L. citriodora oil (CC 50 : 139.5 vs. 57.6 µg/mL). Virus plaque reduction for all four dengue serotypes was observed by treatment of the virus before adsorption on cell. The IC 50 values for L. alba oil were between 0. 4-32.6 µg/mL and between 1.9-33.7 µg/mL for L. citriodora oil. No viral inhibitory effect was observed by addition of the essential oil after virus adsorption. The inhibitory effect of the essential oil seems to cause direct virus inactivation before adsorption on host cell.Key words: essential oil -antiviral -virucidal activity -dengue virus -Lippia sp.Dengue is an acute illness characterised by the hemorrhagic fever caused by dengue virus (DENV) that is transmitted to humans by Aedes mosquitoes. DENV infection results in a spectrum of disease ranging from a mild febrile illness or dengue fever to a severe disease or dengue hemorrhagic fever/dengue shock syndrome. Among the human arboviral diseases, dengue is responsible for causing most illness and death. It has been estimated that 50-100 million dengue infections occur each year in tropical urban areas around the world with 20,000-25,000 deaths (Gubler 2006).DENV is a member of the Flaviviridae family and it exists as four related serotypes (DENV-1 2, 3 and 4). The virus possesses an icosahedral nucleocapsid core surrounded by a host-derived lipid membrane (envelope), in which the envelope (E) protein and membrane (M) protein are embedded. The Flavivirus enters into the host cell by receptor-mediated endocytosis into coated vesicles, a process in which the E protein plays the major role. The E protein mediates interactions within the virus and the components of the host cell membrane involved in virus adsorption and the E protein interacts with the membrane cell during fusion (Lindenbach & Rice 2007). Compounds that interact with sites on the E protein that binds to structures on the cellular membrane affect the adsorption and fusion process. That interferes There is no antiviral drug for treatment for any of the Flavivirus and an effective vaccine for human use is not yet available to prevent dengue. Consequently, the development of antiviral drugs licensed for treatment of patients remains an urgent need to prevent dengue fatalities. Compounds obtained from traditional medicinal plants and herbs species have been reported to have antiviral activity and a wide variety of active phytochemicals hav...