The presence of the recombinant cp4 epsps gene from Roundup Ready (RR) corn and RR soybean was quantified using real-time PCR in soil samples from a field experiment growing RR and conventional corn and soybean in rotation.  RR corn and RR soybean cp4 epsps persisted in soil for up to 1 year after seeding.  The concentration of recombinant DNA in soil peaked in July and August in RR corn and RR soybean plots, respectively.  A small fraction of soil samples from plots seeded with conventional crops contained recombinant DNA, suggesting transgene dispersal by means of natural process or agricultural practices.  This research will aid in the understanding of the persistence of recombinant DNA in agricultural cropping systems.