Injuries caused by the western bean cutworm, Striacosta albicosta (Smith), on transgenic Cry1Ab Bacillus thuringiensis (Bt) corn hybrids were documented and quantified. The western bean cutworm is an emerging or potential pest of transgenic Bt corn in South Dakota. The proportion of ears infested with western bean cutworm larvae in the Cry1Ab Bt corn hybrids were 18–20, 38–70, and 0–34% in 2000, 2003, and 2004, respectively. The Cry1Ab Bt corn hybrids were almost completely free of European corn borer infestations. Untreated conventional corn hybrids were less infested with western bean cutworm larvae but more infested with European corn borer larvae. The proportion of ears infested with European corn borer larvae alone were 33, 58–80, and 8–25% in 2000, 2003, and 2004, respectively. Infestations with western bean cutworm alone were 28, 8–28, and 13–19%, respectively. Proportion of ears simultaneously infested with both western bean cutworm and European corn borer larvae were much lower than single infestations by either species alone, indicating niche overlap and competition. Simultaneous infestations by the two species on untreated conventional corn hybrids were only 8, 0–18, and 0–1% in 2000, 2003, and 2004. The corn grains harvested from injured ears were also analyzed for fumonisin and aflatoxin through quantitative enzyme-linked immunosorbent assays. More mycotoxins were found in 2003 when the levels of insect infestation in the corn ears were higher than in 2004. Results from this study underscore the need to investigate other emerging or potential arthropod pests of transgenic Bt corn hybrids in addition to the western bean cutworm.