The cultivars 'Tamcot CD3H' and 'Tamcot CAB-CS' grown in the Brazos River valley near College Station, Texas, in 1988 and 1989, respectively, were treated with the biocontrol agents, Bacillus subtilis Bacillus thuringiensis, Pseudomonas fluorescens, or Saccharomyces cerviseae, or with variable rates of nitrogen fertilizer (0, 60, 120, and 180 lb o N/acre). The cultivar 'Tamcot CAMD-E' grown at the Texas Agricultural Experiment Station at Corpus Christi, Texas, in 1988 was treated with three rates o a 2:1 mixture of the biocontrol bacteria, Bacillus cereus and Bacillus megaterium. The biocontrol agents were applied as sprays of buffered live cell suspensions at two-week intervals beginning 2-4 wk after initial flowering at College Station and as a single spray at the onset of the one-third grown flower bud stage at Corpus Christi. At College Station in 1988 flowers were tagged 3-4 days after each of the first two sprays, and bolls from these flowers were harvested just before or after defoliation. Biocontrol treatments did not significantly affect bacterial populations or lint yield at either location in either year. Bolls set early had significantly (P=0.01) lower concentrations and percentages of gram-negative bacteria than those set late. Concentrations of total and gram-negative bacteria after defoliation were threefold greater than those before defoliation (significant at P=0.01). In 1988 numbers of total and gram-negative bacteria at 180 lb of N/acre were significantly greater (P=0.05) than those at 60 or 120 lb of N/acre; and concentrations of gram-negative bacteria were correlated with both rates of fertilization (r=0.85) and nitrogen contents of fiber (r=0.40). Similar differences did not occur in 1989 when insect damage was high and yields were low in all plots. We conclude that biocontrol agents currently approved for use on cotton are not useful for reducing populations of toxic gram-negative bacteria, but improved management of fertilizers, defoliants, and early season insects should reduce numbers of gram-negative bacteria on fiber.