- Advantage is taken of a unique spectral feature of soil biogenic crust containing cyanobacteria. It has been shown that the special phycobilin pigment in cyanobacteria contributes in producing a relatively higher reectance in the blue spectral region than the same type of substrate without the biogenic crust. A spectral crust index (CI) has been developed, based on the normalized diÄ erence between the RED and the BLUE spectral values: CI=1Õ (REDÕ BLUE)/(RED+BLUE). Applying the index to a sand dune environment, it has been shown that the CI can be used to detect and to map, from remote sensing imagery, diÄ erent lithological morphological units, such as, active sands, crusted interdune areas and playas, which are expressed in the topography. As a mapping tool the CI image is much more sensitive to the ground features than the original images. The absence, existence, and distribution of soil crust are an important information for deserti® cation and climate change studies. They are also highly valuable information for developing agricultural regions and/or infrastructures in arid environments since soil crusts contribute to soil stability, soil build-up, soil fertility, and to the soil water regime. The application of the proposed CI can be performed with imagery acquired by any sensor which contains the blue band. Currently, the most common data sources are colour aerial photographs and Landsat TM images as demonstrated in this paper. However, CI should be applicable to other sensors such as the SPOT- VEGETATION, MOMS-2P, SeaWiFS and MODIS which will be available in the coming years.