Significant increases in growth and yield of potato plants were achieved by treating seedpieces with suspensions of 2 Pseudomonas spp. at .apprx. 109 colony-forming units (cfu)/ml prior to planting. The pseudomonads were selected from over 100 strains that were isolated from the surface of potato tubers and also exhibited antibiosis against Erwinia carotovora var. carotovora in vitro. The isolates were identified as strains of P. fluorescens and P. putida. These strains survived for at least 1 mo on treated seedpieces planted in loamy sand field soil at populations of .apprx. 109 cfu/0.785 cm2. Also, they colonized developing potato roots and were the predominant bacteria in the rhizospheres up to 2 mo after planting. Bacterization of seedpieces planted in field soils in the greenhouse resulted in up to 100% increase in fresh wt of shoot and root systems in a 4 wk period. Statistically significant increases in yield ranged 14-33% in 5 of 9 field plots in California and Idaho [USA]. The pseudomonads had no effect on plant growth or tuber yield when seedpieces were planted in peat soil, or in soil that was relatively dry. Both Pseudomonas spp. were compatible with fungicides that were commonly used to treat seedpieces, except for manganese ethylenebisdithiocarbamate zinc salt (mancozeb). The mechanism by which these bacteria enhance plant growth and tuber yield may be associated with changes in the composition of rhizosphere bacterial flora.