Pistia stratiotes L. (water lettuce) reportedly does not produce fruits and seeds in the United States. However, we discovered water lettuce fruits and seedlings at Loxahatchee National Wildlife Refuge, Palm Beach County, Florida. In April 1987, 95% of this population consisted of seeds. Forty percent of the 267 ramets m−2 of mat held fruits and the mat averaged 726 seeds m−2. Sediments held 4196 seeds m−2. Germination experiments showed that over 80% of the seeds were viable, a much greater proportion than reported in Brazil. Since the initial discovery, we have found seeds at many sites throughout south Florida. We have concluded seed germination is an important factor in the dynamics of some waterlettuce populations in the United States.