Broad-host-range plasmids, belonging to IncP (RP4 and pUPI102) and IncC (R57.b), were studied for intrageneric and intergeneric gene transfer in three different soil microcosms. RP4 was transferred intragenerically in clay loam, sandy loam, and sandy microcosms at frequencis of 0.71 x 10(-2), 0.83 x 10(-2), and 0.41 x 10(-2) respectively, optimally at 37 degrees C and at 100% vol/wt moisture content. Under similar conditions, R57.b was also transferred at frequencies of 0.38 x 10(-2), 0.58 x 10(-2), and 0.80 x 10(-5) respectively at 30 degrees C. Both RP4 and R57.b were transferred at low frequency at 20 degrees C. Kinetics of plasmid transfer revealed that 48 h was the optimum time for intrageneric conjugal gene transfer. Gene transfer frequency was tenfold higher in all nutrient-amended soil microcosms than in the absence of nutrient amendment. RP4 was transferred to an indigenous soil bacterium Beijerinckia indica in a nonsterile soil microcosm and to other indigenous soil bacteria, viz. Xanthomonas campestris, Azotobacter chroococcum, Acinetobacter calcoaceticus, Achromobacter agili, and Rhizobium meliloti in sterile soil microcosms. pUPI102 was transferred fromA. calcoaceticus BD413 to Escherichia coli K12 J53 at a frequency of 0.75 x 10(-6) and 1.1 x 10(-6) in clay loam and sandy loam microcosms respectively. However, no gene transfer was observed in any soil microcosm when strains of A. calcoaceticus BD413 (pUPI102) and E. coli K12 J53.2 (RP4) were used for conjugal mating. Plasmid RP4 was found to be 100% stable in all the above microorganisms.