BACKGROUND: Acorus gramineus is of medicinal plants that exhibit variable biological activities for human health and against insect pests. The extracts of A. gramineus was examined in an attempt to develop a natural repellent against human disease-mediating Culex pipiens. METHODS AND RESULTS: The roots of A. gramineus dried under dark conditions were homogenized and extracted with ethanol. The extracts were subjected to repellent activity assays against C. pipiens in a hand-made acrylamide box with three accessible rooms. Significantly low number of mosquitos was found in the room previously fumigated with the extracts at 50 mg/L on the filter paper, exhibiting less than 20% of mosquitos tested. More than 50% of mosquitos tested was found in the room without the extracts, but less than 30% was found in the room that released mosquitos. GC/MS analysis detected β-asarone as a main component of the extracts. The commercial asarones (α and β) showed a repellent activity at 50 mg/L on the filter paper similar to the extracts. CONCLUSION(S): A. gramineus has potential for use as a mosquito repellent since β-asarone, a main component of the plant, exhibited a strong repellent activity against C. pipiens.