In certain localities, R. minor and R. serotinus grow sympatrically and the flowering-periods overlap. The species hybridize but can still be recognized as distinct taxonomic entities. In the field s x m crosses can be expected to occur more frequently than the reverse, on the basis of flower morphology and pollinator (bumblebee) efficiency. Observation of pollen germination, pollen tube growth, seed set, and seed germination in artificial, reciprocal crosses permits the conclusion that a single m x s pollination leads to more offspring than a single s x m pollination. The two species are isolated from each other by a series of mechanisms none of which is 100% effective by itself, but their combined action comes close to that figure. The leakages in the ethological barrier against hybridization are closed, partly, by physiological mechanisms.