Relevance. The influence of crop rotations on the infestation of crops has been little studied. А goal was to investigate the influence of biologized crop rotations on infestation of crops in the arid steppe of the Trans-Ural region of Bashkortostan.Methods. The relief of the experimental field is flat, represented by the foothill steppe of the Southern Urals. The soil is black soil, common medium-coal. The climate is arid, with periodically hot summer and moderately cold and little snow winter periods. During the years of experiments (2019–2021), the amount of precipitation for the vegetation period, May — August, averaged 88 mm. Over the past 10–15 years, the aridity and unevenness of the distribution of atmospheric precipitation has noticeably increased, which led to the oppression of field crops and the complication of the fight against weeds.The method of field experimentswas applied. Accounting of infestation was carried by quantitative and weight method. The effect of biologized crop rotations with cereals and alfalfa on the degree ofinfestation of crops compared to traditional grainfallow rotation was evaluated.Results. The infestation of crops in experimental crop rotations depended on the strength of the development of cultivated crops and suppression of weed plants. In the favorable conditions of 2019, weeds developed poorly in well-developed crops, with an amount of 8–10 piece /m2 in biologized crop rotation. In grain-fallow crop rotation, the infestation was 1.2–1.9 times greater. On average over three years at the beginning of the field crop vegetation there are 14–19 piece /m2 weeds in biologized crop rotations, which is 45–56% less than in grain-fallow crop rotation. Before harvesting, the difference in infestation between crop rotations is reduced to 12–29%, but the advantage in reducing infestation in biologized crop rotations remains. In more humidi fied years, fertilizers contribute to a decrease in infestation compared to a back ground without fertilizer. In especially dry years, fertilizers lead to some increase of infestation of crops. Thus, it was found that biologized crop rotations effectively reduce the infestation of crops, regardless of the background of fertilizer.