Summary: A method for studying plasmid transfer in the rhizosphere is described. This work demonstrates plasmid transfer in an unenclosed rhizosphere under field conditions. The donor (Pseudomonas marginalis 376N) and recipient (Pseudomonas aureofaciens 381R) bacteria and the conjugative mercury resistance plasmid (pQBR11) studied were all isolated from the bacterial community indigenous to sugar beet rhizosphere. Spontaneous nalidixic acid and rifampicin resistant mutants of these bacteria were used as donors and recipients of pQBR11 for in situ matings. Fresh field soil was mixed with donors and recipients to give a soil mating mix (SMM) which was placed underground on the surface of a sugar beet root storage organ. Plasmid transfer in the SMM was determined after 24 h at frequencies between 5·1 × 10−5 and 1·3 × 10−8 transconjugants per recipient. Higher transfer frequencies (1·3 × 10−2 to 1·7 × 10−6) were recorded on the peel adjacent to the SMM. No transfer of mercury resistance was detected in SMM controls incubated at 20°C in vitro or placed in soil at distances of more than 5 cm from plants.