Polygonum aviculare is a widely distributed annual herb. Number of segregates from this highly variable taxon has been described as independent species or different subspecies on the basis of habit, heterophyllous and homophyllous leaves and the length of perianth tube. Most proposed subspecies are partially sympatric and their distributions have been affected greatly by humans. Our studies show that P. aviculare has two subspecies in Iran. These infra-specific taxa can be differentiated by length of perianth tube and some features of outer tepals.