The experiments were carried out in 2001–2004. The aim of the research was to establish such chlorsulfuron dose that would be effective for Apera spica-venti control and would not be phytotoxic for winter wheat. Besides, it was performed to identify resistance of A. spica-venti biotypes appearing in the experimental fields to chlorsulfuron. The field experiments were conducted in winter wheat that were notably of A. spica-venti weedy. The fields belonged to a private farm at Karczyce and Gałów. The cereals had grown there for a long time and winter wheat monoculture together with repeated chlorsulfuron application was used. The experiments were conducted using randomized blocks method with 3 reapplications. Chlorsulfuron (Glean 75 WG) was applied at rates ranging from 1 to 4.5 times higher than the recommend field dose (15–67.5 g a.s./ha). The degree of sensitivity of A. spica-venti to chlorsulfuron was established by biological tests in greenhouse conditions. The herbicide was applied at four – leaf stage of development at rates ranging from 1 to 32 times higher than the recommend field dose (11.25–360 g a.s./ha). Poor chlorsulfuron efficacy to control of A. spica-venti or its lack might testify for resistance of this species to the herbicide. Chlorsulfuron as an active ingredient was very selective for winter wheat. An application of 67.5 g/ha of the herbicide was not harmful to growth and yielding of winter wheat. Unfortunately, even this dose of the herbicide was ineffective in control of A. spica-venti. In fields with a long-term monoculture of winter wheat together with the use of chlorsulfuron, resistance of A. spica-venti biotypes to this active ingredient was ascertained. Studies in greenhouse conditions proved that the biotypes were not damaged even when the dose was exceeded 32 times. The above confirmed the resistance of these biotypes to chlorsulfuron.