In a field study conducted around Lake Manzala (Egypt) during 1995-1996, the distribution of the floating-leafed lotus (Nymphaea lotus L.) in irrigation canals and ricefields was analyzed by classification and ordination methods. Environmental factors associated with its abundance and distribution were evaluated. A data set of 30 stands of lotus and nine environmental variables was analyzed by Canonical Correspondence Analysis (CCA) ordination relating environmental factors to the occurrence of the weed. Ordination analysis detected a strong association between soil organic matter and fine sediment (silt + clay) in the distribution of lotus. Water variables were less important. The density of lotus averaged 3.3±0.49 plants m-2 in the irrigation canals and 6.6±1.4 plants m-2 in the ricefields. The phytomass of lotus ranged from 877 g m-2 in the pure stands to 226 g m-2 in ricefield habitats. The highest proportion of lotus phytomass was allocated to leaf blade, with values reaching 40% of total phytomass in the infested ricefields. Allocation proportions to other plant organs varied between natural and ricefield habitats. Lotus plants allocated more dry matter to vegetative reproduction than to sexual reproduction. The ratio of vegetative to sexual reproduction ranged from 3.48 in the pure stands to 9.63 in infested ricefields. Populations of lotus showed wider variation in leaf area classes in pure stands than in rice stands. Presence of lotus as a weed in ricefields significantly reduced the growth and grain yield of the crop, indicating the risk of its spreading as an invasive weed.