Crataegus monogyna Jacq. is an old world species that has become naturalized in parts of North America. Crataegus punctata Jacq. is a native species whose range is restricted to eastern North America. Both taxa are diploid, obligately outcrossing species that are aggressive colonizers. In southern Ontario, they are the most common sympatric species pair within the genus and have overlapping floral phenologies. Hybrid individuals occur sporadically throughout sympatric populations and are morphologically intermediate with respect to the parental taxa. Experimental crosses were performed and characteristics of the pollination biology of each species were examined: pollen production and viability, pollen tube growth and pollen:ovule ratios. Polymorphic alleles were identified with DNA microsatellite markers. The results collectively demonstrate that there are weak ecological and mechanical barriers to gene flow between these species.