The interaction of Rhizoctonia solani and Meloidogyne incognita Race 3 and Race 4, was tested on four cultivars of American cotton namely RNR-120, RNR-315, DP-50 and Auburn-634.The obtained data revealed that the artificial infection to the susceptible cultivar DP-50 with the fungus in combination with either nematode races increased the root galling from 25-30% in case of nematode alone to 30-40% respectively. Combined infection also increased the pre-emergence damping-off from 60% in case of fungal infection alone to 80%. At the same time the root necrosis of the same cultivar was increased by 25-30% in the combined infection. The second susceptible cultivar to nematode (RNR-315) gave almost similar results in which the pre emergence damping-off was increased from 40% in fungal infection alone to 80% in the combined infection with either races 3 or 4 of the nematode. However, galling and root necrosis were not affected by combined infection.Both cotton cultivars RNR-120 and Auburn-634 were resistant to nematode infection. On the other hand, the pre-emergence damping-off of the cultivar RNR-120 was increased from 80% to 100%, when the race 3 was combined with the fungus. Galling and root necrosis were increased from 10% to 30% under the combination between the fungus and either nematode races 3 and 4. Also, the results showed that cv Auburn-634 was resistant to both nematode races but susceptible to the fungus. Such data revealed that the synergistic effect is limited to susceptible cotton cultivars to nematodes, whereas the nematode resistant cultivars did not affected much by the combined infection of the fungus plus nematode.