Summary 0[ The individual and joint e}ectiveness of two biorational tactics "crop interference and exploitation of negative cross!resistance to certain herbicides# in the management of triazine!resistant Amaranthus hybridus L[ "smooth pigweed# were estimated[ Bio! rational tactics exploit biological idiosyncracies of resistant "R# genotypes to maximize _tness cost"s# of resistance[ We quanti_ed selection against triazine resistance by relative performance comparisons between lines having comparable nuclear genomes but either resistant or susceptible cytoplasm[ Increasing soybean density by reducing row spacing "from 65 cm to 14 cm# did not signi_cantly increase the _tness cost of resistance[ 1[ Low doses of bentazon "099 and 299 g active ingredient ha Ð0 # did strongly increase the cost of resistance[ Over 1 years\ the mean relative performance of R genotypes in bentazon treatments was 9=39\ compared to 9=59 in the absence of bentazon[ Therefore\ use of bentazon in soybean production has the potential to delay evolution of triazine resistance in maizeÐsoybean rotations using triazines[ 2[ There was no consistent indication that increased soybean density and bentazon herbicide could act synergistically to increase costs of triazine resistance in Amaranthus hybridus[ Nor were di}erences in response to biorational tactics evident between the two populations of origin "Maryland and Virginia\ USA# from which experimental lines were derived[ 3[ E}ects of the biorational tactics di}ered markedly between years\ highlighting that resistance management depending primarily on these tactics would have widely variable results[ Use of such tactics is likely to be most e}ective in the context of diversi_ed weed management[ Key!words] herbicide resistance\ negative cross!resistance\ resistance management\ Amaranthus hybridus\ weed evolution[ Journal of Applied Ecology "0888# 25\ 012Ð021