Unreduced gametes, the primary cause of polyploid formation, are considered maladaptive; however, their frequency and genetic variation among natural populations is rarely measured. Using the perennial, mixed-ploidy plant, Chamerion angustifolium, I investigated the magnitude and two sources of variation in unreduced gamete production: elevation and gamete sex. I collected seeds from three high and three low elevation diploid populations in Kananaskis, Alberta and estimated unreduced male and female gamete production in a greenhouse environment using reciprocal diploid-tetraploid cross-pollinations and flow cytometry. Of all fruit tested, 11.5% produced ≥1 seed through unreduced gametes (4x), with a mean percentage of 0.72% per plant. High elevation populations had a higher percentage of fruit with 4x seed but a lower percentage of unreduced gametes per fruit than low elevations. Male and female unreduced gamete production were affected differently by elevation. These results indicate that genetic variation for unreduced gametes is maintained in natural populations.