GUS2000 and GUS376 are strains of the bacterium Bacillus subtilis which have shown activity against seedling disease organisms occurring in cotton. In field tests conducted in 1992 with five cultivars, both bacteria colonized the root systems of cotton plants throughout the entire growing season. Different cultivars provided significantly different levels of colonization by either isolate. However, differing colonization on cultivars could not necessarily be attributed to cultivar attributes, since initial seed pH of the cultivar was significantly correlated with colonization as well. GUS376 colonized lateral roots of the cotton plant significantly better than GUS2000 (P<0.05). However, both bacteria colonized the taproot at similar levels. Higher colonization levels occurred in the upper 5.0 cm of the taproot. Use of combinations of the two bacterial strains with each other and/or Trichoderma and Gliocladium fungal biocontrol agents did not result in improvements in cotton plant stand, disease ratings, or yield. However, individual treatments with either bacterial strain did provide improved control of seedling disease and promotion of season-long root health. As indicated by soreshin ratings, GUS376 provided for lower soreshin severity (P<0.05) than did the commercial fungicide seed treatment alone. In general, plant emergence and stands were not significantly increased with either bacterial inoculant. However, in one test using cultivar DPL50, addition of GUS2000 did result in a significant improvement in stand. Additionally, DPL50 in the same test showed an improvement in yield (P<0.07). GUS2000 also showed activity against Fusarium. Disease incidence was reduced in plots treated with moderate to high rates Of GUS2000. The GUS2000 medium rate (2.5 X lo" bacterial spores per cwt of seed) resulted in a significant reduction in damage due to Fusarium (P<0.06).