Onion (Allium cepa L.) is the most important cultivated vegetable crop of India. The total area under onion cultivation in India is about 2,17,340 acres while Bihar alone covers an area of about 21,110 acres. Two main crops of onion are grown each year viz.; "Set Onion" and "Seedling Onion". Seedling onion is planted during December- January and harvested during April-May in Bihar. Onion is infested by several insect pests, of which onion thrips, Thrips tabaci L. is most destructive. Present investigation was carried out to determine the degree of resistance in different varieties of seedling onion against onion thrips, Thrips tabaci L. in Bihar. On the basis of the result of present investigation, it is evident that Spanish white variety of onion was most resistant (11.25% infestation) where as Poona red variety of onion was most susceptible (38.50% infestation).