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The relative effects of hydrologic disturbance, abundance of root parasites, and ambient physicochemical conditions on total biomass, biomass allocation, and architecture of the emergent macrophyte Justicia americana (L.) Vahl. (Acanthanceae) were investigated in six south-central Alabama streams, USA. Justicia total biomass was negatively related to shading by riparian vegetation, which accounted for 45 and 31% of the total variation at the beginning and end of the growing season, respectively. Contrary to our predictions, total Justicia biomass was unrelated to disturbance by flooding as indicated by the likelihood of streambed movement by bankfull discharge, nor was biomass allocation to below-ground structures positively related to likelihood of substrate movement. Stem density at the end of the growing season was negatively related to abundance of nematode-parasites, but total Justicia biomass was unrelated to parasite abundance. This macrophyte appears to persist in the dynamic environment of eastern North American streams by firm anchorage of below-ground structures and high capacity for regeneration following storm flows. (C) 2004 Elsevier B.V. All rights reserved.