Abstract: This study examined the susceptibility of Amaranthus cruentus Linn. to phytotoxic effects of Tithonia diversifolia (Hemsl) A. Gray, compared the phytotoxicity of the plant parts of T. diversifolia and determined the effect of drying on the phytotoxic activity of this weed. Results showed that the germination, growth parameters and fresh and dry matter production of Amaranthus cruentus were retarded by all the four different aqueous extracts applied. The retardation was more pronounced in older plants. The degree of retardatory effects of the aqueous extracts were found to follow this order: fresh shoot aqueous extract (FSE) > dry shoot aqueous extract (DSE) > fresh root aqueous extract (FRE) > dry root aqueous extract (DRE). There was a significantly higher phytotoxic potency at p<0.05 in the fresh tissue aqueous extract compared with the dry tissue aqueous extract as well as in the shoot aqueous extract compared to the root aqueous extract. Tithonia diversifolia was found to be an allelopathic weed with water-soluble allelochemicals in its plant parts and had such phytotoxic potency that could suppress the growth and nutrient accumulation of associated crop plants.