A computer-based analysis was conducted to assess the characteristics of microsatellites in transcribed regions of rice and Arabidopsis. In addition, two mammals were simultaneously analyzed for a comparative analysis. Our analyses confirmed a novel plant-specific feature in which there is a gradient in microsatellite density along the direction of transcription. It was also confirmed that pyrimidine-rich microsatellites are found intensively near the transcription start sites, specifically in the two plants, but not in the mammals. Our results suggest that microsatellites located at high frequency in the 5'-flanking regions of plant genes can potentially act as factors in regulating gene expression.