Two experiments were performed from 1985 to 1987 at Barranquitas in a Humatas soil to evaluate Ametryn (Evik 80 W) for preemergence control of weeds in Peruvian carrot or arradacha (Arracacia xanthorrhiza Banc). Five treatments were evaluated: Ametryn at 4.98, 8.97 and 17.94 kg ai/ha; a hand-weeded check (hoeing); and non-weeded check. There were significant differences (P ± 0.05) in the yield of commercial roots in the first experiment only between the non-weeded plots (7.00 t/ha) and Ametryn plots at 8.97 kg ai/ha (21.68 t/ha) whereas in the second trial, differences were observed between the hand-weeded (14.18 t/ha) and the non-weeded plots (4.11 t/ha). A considerable reduction in the second experiment was probably due to the fact that plants were harvested two months earlier (at 7 months) than in the first experiment, and some root damage was due to rats. Mean yields were 16.68 and 9.78 t/ha of commercial roots in experiments 1 and 2, respectively. These results showed that weed competition can reduce yields in arracacha by more than 60%. Good to excellent weed control with Ametryn was obtained at 12 weeks, when the arracacha foliage should have covered all soil surface area. There was 35% (first experiment) and 0.53% (2nd experiment) of weed coverage in the plots where Ametryn was applied at the lowest rate, 4.48 kg ai/ha. Herbicide efficiency increased with higher rates in both experiments. No toxic effects on the plants were observed from evaluations made 4 and 8 weeks after the treatments. Data obtained from these experiments showed that Ametryn was highly efficient in controlling weeds in arracacha up to 12 weeks even at the lowest application rate. The information obtained is valuable for obtaining registration of this pesticide for this crop.