Field experiments were conducted to assess the yield loss caused by seasonlong interference of selected weeds in lowland potato. Naturally occurring weed mixtures and densities were used. Potato yield loss due to the interference of Euphorbia heterophylla (1 plant/m2), Phyllanthus spp., Boerhavia spp. (0.5 plant/m2 each), Portulaca oleracea (7 plants/m2) or Eleusine indica (2 plants/m2) was not significant. Parthenium spp., Amaranthus spp. and Rottboellia exaltata (5 plants/m2 each) caused 55, 50 and 28% yield reduction, respectively. Mixtures of Parthenium, Amaranthus and Euphorbia (mixture 1) reduced potato yield by 70%, Rottboellia, Eleusine and Euphorbia combined (mixture 2) caused a 34% yield decrease, and mixed stands of Phyllanthus, Eleusine, Boerhavia (mixture 3) and Portulaca reduced potato yield by 14%. Results indicate that interactions among the weed species in mixtures 1 and 2 occurred, since yield losses caused by weed mixtures do not follow an non-interactive (neutral or additive) model of yield reductions, whereas mixture 3 did.