Although burr medic (Medicago polymorpha L.) is commonly considered a self-pollinating species, intrapopulational variation for morphological, biochemical and molecular markers is relatively high. To investigate whether part of this variation is the result of outcrossing, we designed RAPD analysis experiments to reveal both inter- and intra-accession crossing. No cases of inter-accession hybrids were documented, but an intra-accessional crossing rate of 0 to 4% was estimated for one of the four accessions studied. Therefore, a rare outcrossing event between local individuals and migrating genotypes may contribute to high genetic variability observed in natural M. polymorpha. A better understanding of the factors which influence outcrossing in M. polymorpha is pertinent both for medic breeding programmes and for assessing risks associated with releasing transgenic herbicide-resistant crops.