The productivity of double- and triple-rice cropping conducted at IRRI over the last 30 years has been shown to be declining (Cassman et al 1995). Although extensive studies on various aspects such as soil nutrient supply, deficiency and/or toxicity of nutrients, and climate change over time have been carried out to identify the causes, the phenomenon has not been fully understood. Some toxic substances, accumulated under anaerobic conditions after amendment of plant residues, have been reported to inhibit root elongation (Tanaka et al 1990). Appropriate development and function of root systems are essential for satisfactory yield performance of the crop. This study was conducted to determine the presence of a rooting inhibitor in a soil solution collected from IRRI fields subjected to intensive rice cropping.