Late watergrass (LWG) is an important weed of paddy rice in temperate regions, and in California has evolved resistance to most available herbicides. We studied the ecological fitness of herbicide-resistant (R) and –susceptible (S) LWG biotypes, as well as the nature of interference between these biotypes and rice, using both an additive design and a response surface approach combined with treatments to isolate root and shoot interference. Inter-specific interference on rice from both R and S biotypes was 8 and 11 times more intense than intra-specific interference between rice plants, respectively. Although R biotypes were shorter, had less leaf area and above-ground biomass than S biotypes, strong below-ground interactions enabled R plants to be equally suppressive of rice growth as S plants. R plants deployed a greater proportion of their root biomass (0.63 ±0.02 vs. 0.56 ±0.02 for S) in the upper 3.5 cm of soil, which is where most rice roots were placed. Analysis of relative yield totals (RYT ≥ 1) did not detect antagonistic interactions, and since pots were water saturated, we infer that interference was due mostly to competition for soil nutrients. In spite of similar interference abilities between R and S plants and between both biotypes and rice, a comparison of two R and two S biotypes found that R plants exhibited a significant fitness disadvantage by producing a 48-73% lower seed output than S plants. However, a separate comparison of six biotypes found that R plants shattered their mature seed 23-31 days before S plants, which should enable better seed bank replenishment through partial escape of seed removal by harvest operations. Therefore, since R plants are equally damaging of rice yields as S plants, they will require similar levels of weed control; however, their reproductive fitness disadvantage provides opportunities for management if early shattering can be prevented.