High yields with low costs require that sugar beets be kept free of weeds, during critical periods, using labor or chemical treatments. Since the critical periods for this crop in Castilla - La Mancha (Spain) are unknown, the first goal of this study was to determine the effect of early and late competition on yield. The second goal was to determine the critical periods, while taking into consideration the semiarid climatic conditions of this region. Two irrigation farms located in the province of Albacete are dedicated to sugar beet cultivation. These two farms were chosen to carry out the tests March (140,000-150,000 seeds ∙ ha-1) and harvested in October. Two simultaneous and complementary experiments were carried out in each year and farm. Two scenarios were considered with eight different treatments each. In the first one (With Weeds Until - WWU), plots were infested by weeds up to a certain date. In the second one (Free of Weeds Until - FWU), plots were kept free of weeds up to a certain date. For each test, a randomised experimental blocked field was designed and there were four repetitions, each of them containing eight elemental plots (12 m2). Each plot was weeded by hand or weeds were left to grow till a definite date.The results indicated that a 1% loss of yield was reached in the early competition after 14 days, while a loss of 5% was reached after a period of 41 days after it was infested. The results also indicated that in late competition, if a crop is kept clean for 124 days and it is infested afterwards, a 1% loss is reached. However, the loss increases to 5% if the plot is kept clean for 111 days. For a 1% loss the critical period is 110 days and 70 days for a 5% loss.