Methods for wetland identification and delineation require the investigator to determine whether vegetation is hydrophytic. Two widely used techniques for making hydrophytic vegetation decisions involve dominance ratios (i.e., the percentage of dominant species that are rated obligate (OBL), facultative wetland (FACW), and facultative (FAC) and prevalence indices (i.e., the weighfed-average wetland indicator status of all plants present). We sampled 338 vegetation plots on sites throughout the United States and calculated the dominance ratio and a plot-based prevalence index for each plot. We found that hydrophytic vegetation decisions based on the two methods disagreed on 16% of field plots. Analysis of simulated plot data (n=80,000) indicated that frequencies of disagreement increase as vegetation complexity (i.e., number of strata and number of species per stratum) increases. We conclude that the two methods for hydrophytic vegetation decisions disagree too often to be considered equivalent. Additional studies are needed in differnent biogeographic regions and plant community types to determine the conditions under which prevalence indices, dominance ratios, or some other treatment of vegetation data provide more reliable indicators of wetland vegetation.