Present paper showed allelopahtic effects of Tagetes minuta aqueous extracts on seed germination and root hair development. Allelopathy of aqueous extracts derived from T. minuta examined using two test plant species (Lotus comiculatus var. japonicus and Lactuca sativa). The seeds of test species were inoculated in petri dishes containing 0, 10, 50 and 100% aqueous extracts from T. minuta. At day 5, the relative seed germination ratio to control was evaluated, and the development of seedling root hairs was observed through light microscopy. Seed germination of L. comiculatus var. japonicus was significantly inhibited proportional to the concentrations of aqueous extract, but that of L. sativa wasn`t inhibited. The inhibitory allelopathic effect of T. minuta was found in the development and growth of seedling root hairs. It was conduded that the inhibitory allelophatic effects have been to be investigated using various bioassay, for the allelopathy of plant species shows species-specific and organ-specific.