Twenty-eight rice varieties with different countries of origin, maturity and stage of improvement were screened in the laboratory for allelopathic potential against arrowhead. Initial rice-density experiments established appropriate bioassay parameters and demonstrated that arrowhead response to rice root exudates follows a typical dose–response curve. Results from the equal compartment agar method (ECAM) bioassay showed that a range of allelopathic potential exists in rice germplasm. This work, together with other published studies suggests such potential to be species specific. In this study, the degree of root inhibition ranged from 26.6 to 99.7%. The potential allelopathic effect of arrowhead on rice seedlings was determined to be negligible.