Survival of Pseudomonas fluorescens R2f containing either the conjugative plasmid RP4 or the nonconjugative plasmid pRK2501, and stability of the plasmids were studied in two soils, Ede loamy sand and Guelph loam, and in extracts prepared from these soils. In sterile soils, the introduced bacterial populations initially increased and then remained stable over a 47-day period. The presence of wheat roots did not significantly influence bacterial numbers in Guelph loam, whereas a slight increase occurred in Ede loamy sand. In Guelph loam, both plasmids were stably maintained in the introduced populations, but in Ede loamy sand substantial plasmid loss was observed. The presence of added phosphate in Ede loamy sand enhanced plasmid maintenance in the introduced R2f population. In nonsterile Guelph loam, a slow decline in the introduced populations was noted, regardless of plasmid type, whereas in Ede loamy sand the decline was more rapid. There was no detectable effect of plasmid type on host survival. Both plasmids RP4 and pRK2501 remained present in the R2f populations in these soils. The results obtained with sterile soil extracts substantiated the data on plasmid loss in both soils; both plasmids were rather unstable during starvation in minimal medium. The results indicated the absence of an effect of plasmid type on host survival. Soil type significantly affected host survival and plasmid maintenance, and higher survival and stability were observed in the heavier-textured Guelph loam.Key words: survival, plasmid stability, Pseudomonas spp., soil, microcosms.