An agro-botanical study of Nigerian yams comprising 2946 seedling-derived genotypes and 21 clones of Dioscorea rotundata Poir. as well as 19 cultivars of D. cayenensis Lam. was made from 1976 to 1979 at IITA, Ibadan, Nigeria. Comparison of typical forms of D. rotundata, D. cayenensis and intermediate morphotypes revealed differences in leaf, shoot, tuber and phenological features which account for their distinction, differing agronomic requirements and cultural management. The controversial taxonomic status of Guinea yams was re-examined in light of pollen germination in vitro and cross-compatibility data, together with the morphological and phenological differences. The conclusion is reached that D. rotundata and D. cayenensis are not conspecific but constitute two biological species which share a common secondary gene pool. While the presence of intermediate (presumably hybrid) types is the primary cause of confusion in the taxonomic delineation of Guinea yams, farmers easily distinguish between both yams.