The viability of Pistia stratiotes L. seeds was not affected by a period of two months in water at 4 C while most seeds could even stand a few weeks in ice at 5 C. Germination did not occur below a temperature of cira 20 °C whereas a temperature around 25 C was optimal for germination. There was an after-ripening period of circa 6 weeks. Contradictory to a previous report, the submerged seeds were able to germinate if the light intensity was sufficiently high; seeds did not germinate in the dark. It was concluded that in all probability Pistia seeds would be able to survive the Dutch winter season on the bottom of a waterbody. However, the chances for this weed to become permanently integrated in the Dutch flora are considered to be extremely small, in connection with the rarity of seed formation and the adverse conditions for germination prevailing in this country. In contrast to an earlier report it was observed that growth of Pistia on a modified Long-Ashton medium is strongly inhibited at pH 4 while there is a growth optimum at pH 7. As a consequence the conclusions on the ecology of Pistia in relation to its pH requirements, which repeatedly turn up in the literature, especially with regard to mixed vegetations with Eichhornia crassipes should be reconsidered.