Large scale destruction of mature mandarin orange groves due to root rot has occurred in Central Malaya. During the course of an investigation on the aetiology of this malady, Phytophthora nicotianae var. parasitica (Dast.) Waterhouse was isolated, and a test on the susceptibility of nine common citrus species to Phytophthora root rot was carried out by a modified water-culture inoculation technique. This appears to be an efficient and rapid screening method suitable for tropical conditions. The results showed that none of the species tested was resistant, although the degree of susceptibility varied significantly among species and between groups of species.