In the plant sexual polymorphism tristyly, disassortative mating among floral morphs should result in frequency-dependent selection leading to equal frequencies of the three morphs in populations at equilibrium. Direct evidence for frequency-dependent selection in natural populations is, however, lacking. Here we attempt to detect frequency-dependent selection in rapidly expanding populations of the invasive wetland herb Lythrum salicaria. Deterministic computer models were used to assess the expected change in morph ratios. Model results were tested with data from 24 Ontario populations each with an initially low frequency (<0.11) of one morph sampled over a 5-year interval. On average, morph evenness and the frequency of the rare morph increased significantly between samples (mean frequency change = +0.034; range = −0.045 to +0.278). As predicted by the theoretical model, changes in both morph evenness and the frequency of the rare morph were positively correlated with initial morph frequencies. However, no evidence was obtained for expected correlations with the magnitude of the 5-year increase in population size. The results provide the first empirical demonstration of frequency-dependent selection on morph ratios in natural populations of a heterostylous plant.