Abstract: Laboratory and greenhouse experiments were conducted to determine the allelopathic activity of sunflower (Helianthus annuus) on seed germination, seedling growth, flowering and nodulation of bambara groundnut (Vigna subterranea L. Verdc.). Bambara groundnut seeds were treated with diluted and undiluted sunflower extracts under laboratory conditions. Sunflower was also mixed-planted with bambara groundnut in the greenhouse. The effect of sunflower plant residues on bambara groundnut was also determined. Sunflower leaf and root extracts reduced seed germination, seedling growth and reduced seedling dry weight of bambara groundnut under laboratory conditions. Root length was reduced more than the shoot length. Diluted extracts also inhibited seed germination and seedling growth in a concentration-dependent manner. There was no significant difference in the shoot length, number of leaves, nodules and flowers of bambara groundnut which was mix-planted with sunflower compared to bambara groundnuts grown on their own under greenhouse conditions; except for dry weight which was reduced. However, there was an increase in dry weight of sunflower that was mix-planted with bambara groundnut compared to sunflower grown on their own. Flowering and nodulation was completely inhibited in bambara groundnuts grown in soils mixed with sunflower plant residues at 75:25 and 50:50 (w/w) soil:residue mixture. Seed germination, shoot elongation and leaf formation was also reduced. Similar results were also obtained when the amended soil was re-used to grow bambara groundnut. The results of this study suggest that sunflower contains allelochemicals that have strong inhibitory effect on growth, especially flowering and nodulation, of bambara groundnuts. The results indicate the implications of cropping systems between sunflower and bambara groundnuts.