Aims To investigate the antiviral efficacy of oregano oil and its primary active component, carvacrol, against the nonenveloped murine norovirus (MNV), a human norovirus surrogate. Methods and Results Along with an observed loss in cell culture infectivity, the antiviral mechanisms of action were determined in side-by-side experiments including a cell-binding assay, an RNase I protection assay and transmission electron microscopy (TEM). Both antimicrobials produced statistically significant reductions (P <= 0 center dot 05) in virus infectivity within 15min of exposure (c. 1 center dot 0-log(10)). Despite this, the MNV infectivity remained stable with increasing time exposure to oregano oil (1 center dot 07-log(10) after 24h), while carvacrol was far more effective, producing up to 3 center dot 87-log(10) reductions within 1h. Based on the RNase I protection assay, both antimicrobials appeared to act directly upon the virus capsid and subsequently the RNA. Under TEM, the capsids enlarged from <= 35nm in diameter to up to 75nm following treatment with oregano oil and up to 800nm with carvacrol; with greater expansion, capsid disintegration could be observed. Virus adsorption to host cells did not appear to be affected by either antimicrobial. Conclusions Our results demonstrate that carvacrol is effective in inactivating MNV within 1h of exposure by acting directly on the viral capsid and subsequently the RNA. Significance and Impact of the Study This study provides novel findings on the antiviral properties of oregano oil and carvacrol against MNV and demonstrates the potential of carvacrol as a natural food and surface (fomite) sanitizer to control human norovirus.