Background: Rice seedling blight caused by diverse pathogenic microorganism occurs worldwide, and it is currently the most important seedling disease affecting rice production in Northeast China. Understanding the biological characteristics, host range, and fungicide sensitivity of pathogens causing rice seedling blight has important theoretical and practical significance for further exploring occurrence of the disease and for making targeted prevention and control measures.Results: 10 isolates of Pythium spp. were obtained from rice seedling blight, according to Koch’s postulates, in Heilongjiang Province, China. Based on morphology, molecular biology, and phylogenetic tree analyses, the obtained fungal isolates were identified as P. aristosporum. The biological characteristics of P. aristosporum showed that the optimum fermentation conditions were as follows: temperature, 25°C; initial pH, 6; photoperiod, 24 h light; and carbon and nitrogen sources, starch and glycine, respectively. P. aristosporum causing rice seedling blight had a wide range of hosts and could infect tomato (Solanum lycopersicum), carrot (Daucus carota L.), sweetpotato (Ipomoea batatas), cucumber (Cucumis sativus L.), and zucchini (Cucurbita pepo L.), as well as seedlings of broad bean (Vicia faba L.), red bean (Vigna angularis), snailflower (Lablab purpureus (Linn.)Sweet), soybean (Glycine max), mung bean (Vigna radiata), kidney bean (Phaseolus vulgaris), and hyacinth bean (Dolichos lablab L.); however, it could not infect cabbage (Brassica oleracea L.) and maize (Zea mays L.) seedlings. We also evaluated P. aristosporum sensitivity to five commonly used fungicides and found that fludioxonil (EC50 = 0.0731 μg/ml) was the most effective against P. aristosporum.Conclusion: To our knowledge, this is the first report of P. aristosporum causing rice seedling blight in China. Fludioxonil is a relatively sensitive chemical fungicide against P. aristosporum. P. aristosporum causing rice seedling blight has a good environmental adaptability and a wide range of hosts, which poses a great threat to rice seedling production. Occurrence of the new disease caused by P. aristosporum should give full consideration for the use of proper disease management to protect rice.