The causes of low crossability between Pennisetum species were studied. In the compatible cross P. typhoides x P. violaceum, the pollen tube growth was normal and there was good seed set. In P. typhoides x P. cenchroides, pollen tubes reached the ovary but no seed set was obtained indicating ovarian incompatibility. The stylar incompatibility was found in the crosses of P. typhoides with P. schweinfurthii, P. hohenackeri, P. orientale and P. squamulatum. In crosses P. typhoides x P. polystachyon and P. typhoides x P. pedicellatum, mostly pollen tubes did not grow beyond stigmatic surface and only few reached the style. Stigmatic as well as stylar incompatibilities seem to be the reasons for no seed set in these crosses.