In this study the effects of the herbicide linuron on somatic chromosomes of Helianthus annuus L were investigated. The roots were treated with 30, 120, 240 and 500 ppm concentration of linuron for 3, 6 and 12 hours. The mitotic and phase indexes were determined and chromosomal abnormalities were investigated in both control and test groups. It was found that linuron had a marked mitodepressive action on mitosis. The types of chromosomal abnormalities observed included: fragments, disturbed metaphase, c-mitosis, lagging chromosome, and chromatid bridge. A pronounced toxic effect is observed in concentrations 120 ppm and above its. It is considered that linuron may have genotoxic effects in higher plants.