Isolation and characterization experiments over one growing season on 'Stoneville 825' have demonstrated a distinct alteration, with plant age, of the predominant bacterial biotypes recovered from cotton roots. Isolates from cotton seedlings were mostly Gram negative aerobic rods that did not require growth factors and could use a wide variety of amino acids as sole carbon sources. Roots from mid-season plants yielded equal proportions of these same bacteria plus Gram positive spore-formers and mycelial types. Roots from harvest age plants contained largely Gram positive biotypes with a higher proportion of mycelial isolates. Numbers of anaerobic bacteria remained low throughout the sampling period while facultative types were most numerous on young plants and diminished on older plant roots. The greatest diversity in bacterial genera and metabolic types occurred on young, rapidly growing plants (pre-bloom). With In-planta tests against Pythium ultimum and Rhizoctonia solani, 63% of seedling disease suppression responses were attributed to root/rhizoplane isolates while 37% were observed with rhizosphere isolates. The isolates examined were most active against Rhizoctonia although combinations of isolates were identified that were active against both pathogens.