Summary. A field experiment on the application of the herbicide Lawina 700 SC (the active ingredient metamitron – under different application regimes) was carried out in Priluki near Minsk, Belarus, over the period 2011–2013. It was proved that herbicide application in lacy phacelia crops is not always justified. With intensive growth, the crop plant itself competes with weeds and the application of herbicides is not advisable. It was only in one out of the three years that the weeds delayed and disturbed the growth of phacelia, their harmfulness proved to be significant, and herbicide application had a measurable effect. It turned out that the difference in biological and economic effectiveness between a single and double application of metamitron was insignificant and the phytotoxic effect was not found to increase. The most appropriate herbicide application regime that protected lacy phacelia against annual dicotyledonous weeds was a single application of metamitron (1.4 l·ha-¹) at the 1–2 true leaf pair stage. The study also showed that the herbicide metamitron, applied both before emergence and after emergence of phacelia, may exhibit phytotoxic activity against the cultured crop, slowing down plant growth and causing a decrease in the seed yield. The effectiveness of metamitron application before emergence – both at the rate of 1.4 l·ha-¹ and at a higher rate (2.1 l·ha-¹) – proved to be very unstable, in particular in the aspect of direct phytotoxicity of the herbicide on the crop plant, but also in the aspect of periodic accumulation of the biologically active substance in the soil.