Canopy spectral reflectances were measured over six cultivars of spring wheat (Trticum aestivumL.) grown at Phoenix, Arizona. Data were collected at 30-45 min intervals on 9 March 1983 using two ground-based radiometers with bandpass characteristics similar to those of the Muttispectral Scanner and Thematic Mapper on LANDSAT-4 and -5. Major differences in reflectance were observed among cultivars at every time period despite their apparent similarities in green leaf area and green biomass. Single-leaf spectra measured in the laboratory with a spectrophotometer revealed no cultivar-related differences and supported the contention that the reflectances were strongly influenced by canopy architectural features. The diurnal patterns of reflectance reinforced this conclusion with planophile canopies exhibiting the least amount of variability due to changes in Sun angle and erectophile canopies showing the most. These data underscore the complexities of interpreting remotely sensed multispectral data and suggest that multiple Sun-angle data acquisitions may be required to extract desired information.