A new curcuminoid, cyclocurcumin (IV), was isolated from the nematocidally active fraction of turmeric, the rhizome of Curcuma longa, together with three known curcuminoids, curcumin (I), demethoxycurcumin (II) and bisdemethoxycurcumin (III). The structure of IV was elucidated on the basis of spectral data and confirmed by the partial synthesis from curcumin (I). Although the above curcuminoids were ineffective when they were applied independently, the nematocidal activity increased remarkably when they were mixed, suggesting a synergistic action between them.