Microarray analysis was used to identify changes in gene expression in velvetleaf that result from competition with corn. The plants were grown in field plots under adequate N and irrigation to minimize N and water stress. Consequently only differences due to competition for micronutrients, light and perhaps allelopathic stress were anticipated. Genes involved in carbon utilization, photosynthesis, red light signaling, and cell division were induced when velvetleaf was grown in competition with corn. A less clear picture of the physiological impact of growth in monoculture was provided by the data. However, several genes involved in secondary metabolism and a gene induced by phosphate availability were induced. No differences were observed in genes responsive to water stress or sequestering/transporting.