Septoria leaf blotch is one of the most important diseases of wheat in the world. Severe epiphytotics of the disease occurred in Iran, especially in Khuzestan province during 1996. In an attempt to assess the adverse effects of infections of the disease on yield of 3 main wheat cultivars, a field trial for two years was carried out using a split - plot desing, where the main plots consisted of artificial inoculation at 14-18 day intervals at growth stages GS__12, GS__23, GS__31, GS__37, GS__47&GS__58 corresponding to 17, 33, 51, 66, 81 and 95 days post planting. The subplots were cultivars Falat, Atrak and Darab-2, that were planted in a randomized complete block design in 4 replications. A control of plain water spray was included in the experiments. Indices taken were: grain yield, number of ears per square meter, number of kernels per ear and 1000 kernel weight. Linear regression of mean yield loss and the number days of infection was plotted. Under the conditions of these experiments, yield reduction of 6.99 to 38.20 percent were recorded. The extent of yield reductions depended on the wheat cultivar, its growth stage at the initiation (time) of infection, and the severity of the disease. The disease caused significant reductions in grain number per ear and I000 kernel weight; however, the number of ears per square meter was not affected. Daily reductions of 0.283, 0.249 and 0.301 per cent in yield were caused due to disease in the cultivars Falat, Atrak and Darab-2, respectively.