Self-incompatibility has been demonstrated in two samples of the parasitic flowering plant, Striga hermonthica, originating from Upper Volta and Sudan. Compatibility was determined by fluorescence microscopy of stigmatic preparations following controlled pollinations. Self incompatibility was associated with floral adaptations favouring pollination by long-tongued insects such as butterflies. Outbreeding accounts for the variability of S. hermonthica, both in morphology and in host-specificity. Genetic variation in host specificity imposes limits upon the effectiveness of the present methods of testing tropical cereals for resistance to S. hermonthica.