Sugar beet is commercially grown in Minnesota, Idaho, North Dakota, Michigan, Nebraska, Montana, California, Wyoming, Colorado, Oregon, and Washington [3]. The cultivars are relatively high-yielding and are moderately resistant to most of the common soil-borne and foliar pathogens. Most cultivars have a minimum level of resistance to root pathogens such as Rhizoctonia solani, Aphanomyces cochlioides, Clonostachys rosea, Globisporangium ultimum, Rhizopus stolonifera, and Fusarium equiseti [1, 2, 3, 4]. In June 2019, sugar beet seedlings were observed with hypocotyl and root rot in Sidney (47.7167° N, 104.1563° W), Montana. The disease affected about 20% of the 10 ha commercial field (Figure 1). Hypocotyl of infected seedlings was surface sterilized with 70% ethanol for 30 seconds, rinsed with sterile distilled water, and cultured on 10% water agar (WA) for 7 days at 25°C. Macroscopically, on WA the colonies had profuse white cottony mycelia with black dot-like heads (Figure 2). Microscopically, the sporangiophores developed in groups (3 to 5) simple rhizoids and stolons at their ends (Figure 3). The globose sporangia were large with a size of 90.24 (50.67 to 140.55) µm, and the sporangiospores ranged from ellipsoid to ovoid and measured 5.55 (5.00–8.14) μm × 3.4 (2.92–5.46) μm. Based on the morphological characteristics, the fungus was identified as Rhizopus stolonifer [5]. Pathogenicity assay was done using 8, 2-week old sugar beet seedlings grown individually in pots (4?×4?×6?). Seedlings were inoculated with a mycelial plug which was kept close to the seedlings. Inoculated and control plants were placed in the greenhouse at 25 ± 2°C, and 80% to 85% relative humidity. In 4-5 days, six out of 8 inoculated seedlings showed symptoms similar to those observed in the field and non-inoculated seedlings were symptomless. This study was repeated (Figure 4). The fungus was isolated from diseased roots and confirmed to be Rhizopus stolonifer based on morphological characters. Rhizopus stolonifer and R. arrhizus are the most common fungi mostly found in moist agricultural soils. Rhizopus stolonifer causes disease at low temperatures of 57° to 61°F. Recently, Globisporangium ultimum was reported to cause constriction on hypocotyl of sugar beet seedlings [4]. To our knowledge, this is the first report of Rhizopus stolonifer causing hypocotyl and root rot on sugar beet seedlings in Montana.