In the active delta of the Mississippi River, Phragmites australis occurs in large contiguous stands of predominantly two clonal genotypes. We determined morphological variability among and within clones in this environment. Eight morphological variables were measured on culms from eight clonal populations within three subdeltas. Six populations were from the two dominant genotypes, whereas two populations were of two recombinant genotypes. All eight populations were genetically assessed from previous isozymic analysis. Culm, leaf and panicle morphology varied significantly between the two predominant genotypes, whereas the morphology of the two recombinant genotypes generally fell intermediate to that of both dominant genotypes. Morphological differences found within genotypes among the three subdeltas demonstrated phenotypic plasticity within P. australis. Morphological variability across subdeltas is likely the result of one or several environmental factors related to water depth or substrate quality.