Confusion exists among researchers and turfgrass managers on the type of Pythium root diseases that occur in the Midwest. Twenty-three Pythium spp. were isolated from roots of creeping bentgrass (Agrostis stolonifera L.) putting greens exhibiting summer root decline at golf courses in AK, KS, IL, and MO. Of the twenty-three isolates, eight different species including P. aphanidermatum, P. graminicola, P. oligandrum, P. rostratum, P. torulosum, P. ultimum, P. vanterpooli, and P. vexans were identified using traditional techniques. A greenhouse study was conducted to assess pre- and post-emergent virulence of the identified Pythium species. Cone-tainer flats containing USGA specification sand were seeded with ÔPenn A-1Õ and ÔPenncrossÕ creeping bentgrass and placed in a greenhouse at 30¡C/26¡C (day/night). For pre-emergent virulence assessment, Cone-tainers were inoculated with Pythium-colonized grass blades at a 0.5-cm depth prior to seeding. To assess post-emergent virulence, 6 week-old bentgrass plants were inoculated by cutting the root system at a 5-cm depth, placing Pythium-colonized grass blades onto the surface of sterile sand, and then replacing the turf. Cone-tainers were arranged in a completely randomized design with 10 replications. Disease severity was assessed as a visual estimation of the percent of diseased bentgrass per individual Cone-tainer. Data were subjected to analysis of variance and analyzed using the PROC MIXED procedure in SAS. In pre- and postemergence tests, P. aphanidermatum, P. graminicola, and P. vanterpoolii were highly aggressive causing 61-100% disease severity, P. ultimum was moderately aggressive causing 21-60% disease severity, and P. oligandrum, P. rostratum, P. torulosum, and P. vexans caused ² 20% disease severity. No significant difference in disease severity was detected among cultivars. Symptoms included root and crown rot, blight, and necrosis. Results suggest that infection of roots by Pythium spp. may be a contributor to summer bentgrass decline in the Midwest.