We isolated a small segment of the 3'-untranslated region (3'-UTR) in the mouse connective tissue growth factor (ctgf/fisp12) gene and evaluated its functionality. Comparison of the nucleotide sequences of human and mouse ctgf 3'-UTRs revealed a conserved small segment of 91 bases, The corresponding segments of the 3'-UTRs shared as much as 82.4% homology, whereas the overall homology between the 3'-UTRS was 71.8%. To study the functionality of the conserved segment, the corresponding region of mouse ctgf cDNA was amplified from NIH3T3 cells. When it was fused downstream of a marker gene, it showed remarkable repressive effects on gene expression. The repressive effect of the sense form was more prominent than that of the antisense form. Computer analyses of these sequence predicted stable secondary structures, suggesting that they act at the RNA level. The predicted structures of the sense and antisense forms appeared to be slightly different, which is consistent with the difference in repressive function. These findings defined the conserved small element in the mouse ctgf gene as a potent negative regulator of gene expression, which may act at a posttranscriptional level.