To study the colonization of cotton roots by Pseudomonas fluorescens in sterile soil, germinated cotton seeds were inoculated with approximately 8 x 10⁵ colony forming units (cfu) of P. fluorescens and planted in fumigated soil. Pots were maintained under glasshouse conditions and plants were harvested at weekly intervals for 6 weeks. Rhizosphere and rhizoplane bacteria were assayed from the closely adhering soil and the roots. After one week of plant root growth, P. fluorescens populations were 5 x 10⁶ cfu per g oven dry weight (odw) soil and root in the rhizoaphere while the rhizoplane had 6 x 10⁷ cfu per g odw of root. In the following weeks both populations dropped significantly. The rhizosphere maintained a population of 1 x 10⁵ while the rhizoplane population remained at approximately 1 x 10⁶. There was no significant difference in plant dry weight between treated and control plants. To examine distribution patterns of P. fluoreacens on roots, a rifampicin resistant isolate was used to treat seeds at approximately 10(8) cfu per seed. They were then planted in aluminum soil columns using untreated field soil moistened to 15% w/w. After incubation for 1 week, the columns were carefully sliced open and roots removed and sectioned in 1 cm pieces. These sections were then homogenized and assayed for bacteria on rifampicin and cycloheximide amended media. It was found that 80% of the plants had no P. fluorescens colonization beyond the first 3 cm of root. The first 3 cm had populations of 10(5) to 10(6) cfu per cm root. There were no significant difference in shoot weight between treated and nontreated seedlings.