Abstract.1. We tested the hypothesis that interspecific competition between two grass‐feeding stenodemine bug species (Notostira elongataGeoffroy andMegaloceraeu recticornisGeoffroy) was capable of causing population‐scale mortality in the field.2. N.elongatanymphs were added to two field enclosures in which theM.recti‐cornispopulation was to hatch. In one of these enclosures the grassArrhena‐therum elatiusL., on which onlyM.recticorniscan feed, was also added. Population changes of the bug species were then monitored. An unmanipulated enclosure and an unfenced plot of grassland were used as controls.3. The results demonstrated thatN.elongatalowered the survivalof M.recticornisand that this effect was removed by the addition ofA.elatius. The results also suggested thatN.elongatasuffered less from the effects of competition withM.recticorniswhen the latter had access to the refuge foodplant.4. Interspecific competition was rare among members of the grass‐feeding stenodemine guild in the area studied. Only one out of fifteen possible pairs competed, and this competition occurred only under special circ