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The effect of the time and rate of application of nitrogen on the yield of high yielding varieties of wheat was studied under the semiarid conditions of Northern Tunisia. Analysis of the experimental data showed that nitrogen increased the yield of wheat in most locations which were chosen for this experiment. In high rainfall areas, late application of nitrogen (tillering, jointing stages) was more effective than early applications. The optimum yield was obtained by applying 90 kilograms of nitrogen per hectare. In the medium to low rainfall area, early applications (seeding time) were more effective than the late ones. The optimum yield was obtained by applying 67 kilograms of nitrogen per hectare. The yields varied from one location to another. This variability was found to be very dependent on the amount and distribution of the rainfall, thus moisture in the soil and the residual nitrogen.