Comparison of herbicide-tolerant biotypes with their herbicide-susceptible con-specifics allows us to examine the fitness costs of mutations with a well understood physiological effect. Atrazine-tolerant biotypes of Abutilon theophrasti and Amaranthus retroflexus have contrasting physiological mechanisms for tolerance. In previous studies, we have shown that, under non-competitive conditions, atrazine-tolerant biotypes of Abutilon theophrasti and Amaranthus retroflexus show cross-resistance to the herbicides linuron and metribuzin and show increased sensitivity to the herbicide alachlor. In the absence of herbicides the biomass of the tolerant biotype of A. retroflexus was significantly lower than that of the susceptible biotype, but in A. theophrasti the biomass of the two biotypes was not significantly different. In this study, tolerant biotypes were grown in a DeWit replacement series with their susceptible con-specifics. There were five experimental treatments, four subjecting the plants to sub-lethal dosages of the herbicides alachlor, atrazine, linuron, and metribuzin, respectively and one in which no herbicide was applied. The tolerant biotype of A. retroflexus was superior in fitness, as measured by biomass and seed mass, to the susceptible biotype when treated with atrazine, or metribuzin and, to a lesser extent, when treated with linuron. However, the trend was reversed when plants were treated with alachlor. In A. theophrasti, the susceptible biotype exhibited fitness superiority over the tolerant biotype in the presence of alachlor and to a lesser extent, metribuzin. There was no significant difference between the biotypes in response to linuron and the resistant genotype was superior in the presence of atrazine. When no herbicide was applied, the susceptible biotypes of A. retroflexus and A. theophrasti showed fitness superiority over the tolerant biotypes.

Key words: Abutilon theophrasti, Amaranthus retroflexus, competition, herbicide resistance