Models of polyploid evolution indicate that tetraploids are more likely to establish within diploid populations when they are formed recurrently through the union of unreduced (n=2n) gametes. To account for the coexistence of diploids and tetraploids in populations of Chamerion angustifolium, diploid, triploid and tetraploid plants were crossed in all possible combinations and fecundity and ploidy using flow cytometry of the resulting progeny were measured. Combined with previous data on cytotype fitness, these data were used in a simulation to examine the impact of unreduced gametes on tetraploid evolution. Seed set per fruit was highest in 2x × 2x crosses (69%), intermediate in 4x × 4x, 2x × 4x and 3x × 4x crosses (range, 11–35%) and lowest in 3x × 2x and 3x × 3x crosses (range, 1–10%). Offspring were diploid (94%) or triploid (6%) in 2x × 2x crosses, diploid (17.5%), triploid (56%) or tetraploid (26.5%) in 3x × 2x crosses, and triploid (53%) and tetraploid (44%) in all others (4x × 4x, 3x × 4x, 2x × 4x), indicating that some gametes are unreduced, particularly in triploids. Forty-two percent of offspring, from three different crosses, had DNA contents greater than tetraploids. Computer simulations based on these results showed that unreduced gamete formation via triploids in C. angustifolium can promote the coexistence of diploids and tetraploids, but, due to law triploid fitness, is insufficient to overcome tetraploid minority disadvantage.