Butomus umbellatus L. is an invasive emergent aquatic plant that exhibits wide variation in seed production. Native European populations are fertile and diploid or sterile and triploid. However, adventive North American populations are widely reported to be sexually sterile. We quantified sexual fertility and self-compatibility under greenhouse conditions for nine introduced populations from eastern Ontario, Canada. All populations were highly fertile and self-compatible. Each flower produced an average of 127 ± 7 (mean ± SE) filled seeds and 31.4 ± 0.4% of seeds germinated. This level of fertility is much higher than previously reported, even for fertile native populations. We also quantified the production of seeds, flowers, and inflorescence-borne asexual bulbils in 19 natural populations from eastern Ontario; 17 populations were highly fertile (200 ± 9 seeds/fruit) and 2 produced almost no seeds (0.0-0.3 seeds/fruit), because ovules were either not fertilized or were aborted soon after fertilization. We found no evidence of a trade-off between sexual reproduction and clonal reproduction via bulbils. The wide variation in reproductive strategy observed in these populations raises questions concerning the evolutionary loss of sex in clonal populations, and may have significant implications for the spread and management of this exotic species.Key words: aquatic plants, clonal reproduction, breeding systems, invasive plants, reproductive ecology, sexual sterility.