Five cotton cultivars grown in a Phymatot chum omnivorum field nursery were treated with Bacillus megaterium (SW) and Bacillus licheniformis (isolate 304), a 1:1 mixture of both bacteria, and a control (H₂O). Plants treated with the SW or 304 bacteria had a 5% reduction in percent dead plants due to root rot when compared to the control and the SW+304 mixture. Averaged over cultivars, the bacterial treatments had no significant effect on root length, total root dry weight, tap root dry weight, or proportion of lateral roots in the total system. Plants treated with the SW and isolate 304 had a 24% and 12% increase in total lint yield, respectively, over the control. The SW+ 304 bacterial mixture had no effect on lint yield. The MAR cottons CABU'CS-2-1-83 and MACAOS-3-84 had a lower percentage of plants killed than CDP37HH-1-83, Tamcot CAMD-E and the non-MAR Lankart 57 cultivar. Differences were obtained among cultivars for total and tap root dry weight. Total lint yield of the MAR cultivars was 16-51% higher than the non-MAR Lankart 57. A significant interaction was observed between the bacterial treatments and cultivars. Bacterial treatments of Lankart 57 increased yield an average of 75% and reduced root rot 35% as compared to the control. Similar responses were obtained for Tamcot CAMD-E and CDP37HH-1-83, but with a lesser effect. The bacterial treatments had no beneficial effect on lint yield of CABU'CS-2-1-83 and MACAOS-3-84, or in reducing the incidence of root rot in CABU'CS-2-1-83. An antagonistic effect between B. megaterium and B. licheniformis was indicated.