1.  Gene flow from transgenic crops to feral populations and naturalized compatible relatives has been raised as one of the main issues for the deregulation of transgenic events.  Creeping bentgrass, Agrostis stolonifera L., is a perennial, outcrossing grass that propagates by seeds and stolons.  Transgenic Roundup Ready® glyphosate-resistant creeping bentgrass (GRCB), which is currently under USDA-APHIS regulated status, was planted in 2002 on 162 ha within a production control area in Oregon, USA.2.  We conducted a study to assess transgene flow from the GRCB fields.  A survey within and around the production control area was performed during the year when the GRCB fields produced seed and for 3 years after the fields were taken out of production.  Transgene flow was determined by testing creeping bentgrass and its relatives for expression of the glyphosate resistance transgene.3.  While GRCB seed-production practices were strictly regulated, evidence of transgene flow was found in all years.  In 2006, 3 years after the transgene source fields were taken out of production and a mitigation programme was initiated, 62% of the 585 creeping bentgrass plants tested in situ were glyphosate-resistant (GR).  Our results document not only the movement of the glyphosate resistance transgene from the fields, but also the establishment and persistence of high frequencies of GR plants in the area, confirming that it was unrealistic to think that containment or eradication of GRCB could be accomplished.4.  Synthesis and applications: These findings highlight the potential for transgene escape and gene flow at a landscape level.  The survey provides empirical frequencies that can be used to design monitoring and management methods for genetically engineered (GE) varieties of outcrossing, wind-pollinated, perennial grasses and to evaluate the potential for coexistence of GE and non-GE grass seed crops.  Such information should also be used in the decision-making process for authorization of field trials and deregulation of genetic engineering events.