Parasitic effect of Verticillium dahlia was observed in different phenological stages of two cotton varieties: Deltapine 80 (DP‑80), tolerant, and Nazas 87 (N‑87), susceptible. Both varieties reacted in a similar way to the initiation and different degrees of the fungus attack. When infection occurred between 65 to 142 days after sowing (dds) (822 to 1859 accumulated heat units) with plants being from initial blooming to fruiting yield and height reductions were caused. The severest parasitic impact occurred when the plants were infected 93 dds and there were 1212 heat units. DP‑80 yield was reduced in 96%, while that of N‑87 was reduced by 90%. In general, the yield of the susceptible variety was 2.0 metric ton/ha, while that of the resistant variety was 3.0 metric ton/ha. The difference might be due to the fact that the percentages of infected plants at the critical period of infection were 57% for DP‑80, and 76% for N‑87, and more N‑87 plants presented severe fungus attack at the end of experiment. It was concluded that the parasitic reaction during different phenological stages of both varieties to V. dahliae was similar, but the percentage of infected plants during the blooming stage was higher for the suscept ble variety N‑87 than that of the resistant variety DP‑80.