Stylisma pickeringii var. pattersonii (Convolvulaceae) is endangered in Illinois and Iowa, and occurs in scattered populations in other states. During 1999 and 2000, two insect species previously unreported from Illinois were observed visiting its flowers. This study was undertaken to survey additional insect visitors, as well as to characterize the plant community where S. pickeringii occurs. The objectives were to survey: 1) floral traits (anthesis and flower density) of S. pickeringii, 2) associated plant species, and 3) insect visitor characteristics. Floral traits were determined and associated plant species surveyed in Mason County (degraded hay field on private property) and Henderson County (dry sand prairie at the Big River State Forest), Illinois. Insects visiting flowers were collected at 10:00 a.m., 12:30 p.m., and 3:00 p.m. during June, July, and August in 2001 and 2002. Individual flowers lasted one day and remained open for 6–8 hours. Peak flowering occurred from early to the middle of July when S. pickeringii was the dominant species in flower. Henderson County contained a greater diversity of native plant species with less bare ground and fewer non-native species than the Mason County site. Forty-seven insect species were observed visiting S. pickeringii flowers. Most frequent visitors were Apis mellifera (Hymenoptera: Apidae), Bombylius fraudulentus (Diptera: Bombyliidae), and Heterostylum croceum (Diptera: Bombliidae). The diversity of visiting insects was higher earlier than later in the day, in July and August than June, and in Henderson than Mason County.