To know the action of alachlor plus diuron or cyanazine mixtures to control mono and dicotyledoneus weeds in cotton 'IAC -17' crop, a field experiment was done in 1981/82, in Mogi-Mirim (SP), on sandy clay texture soil, whe realachlor (2 ,15 kg; 2,58 kg and 3,0 kg/ha) was applied in a tank mixture with diuron (1,00 kg/ha) or cyanazine (1,0 kg/ha). Herbicides of the mixtures were also applied alone in those dosages. Hoed and unhoed treatments were also included. All 14 treatments were distributed in randomized bloks, with 4 repplications. Weed infestation observations were done at 14, 31, 45, 75, 82, 94 and 100 days after herbicides applications. When at least one plot had got 25% weed reinfestation, the whole treament was hoed and kept chean till harverting. Plant height, population and cotton production were also considered. The results showed that alachlor (3,01 kg/ha) plus diuron (1,00 kg/ha) gave good control during 100 days period, while alachlor (2,58 kg/ha) plus diu ron (1 ,00 kg/ha) and alachlor (2,58 kg/ha and 3,01 kg/ha) plus cyanazine (1,00 kg/ha) had control action for 94 days period. Alachlor (2,15 kg; 2,58 kg and 3,01 kg/ha) plus diuron (1,00 kg/ha) and alachlor (3,01 kg/ha) plus cyanazine had a tendency to reduce plants height, according analysis results by Tukey test at 5%, while, unhoed check pre sented the lowest plant height. The population and production were not influenced by herbicide treatments, being similar to hoed check and higher tham unhoed check.