Changes in the vegetation of two coastal rivers in Israel were studied in relation to increasing pollution. Pollution levels were defined mainly in terms of ammonium content, presence of detergents and dissolved oxygen concentration at various depths and at various times of the day. It was found that species diversity was greatest in the clean-water sections of the rivers and decreased considerably with increasing pollution. No floating-leaved or submerged species were found in the polluted sections; these and some other species were exclusive to the clean-water sections. On the other hand, none of the hygrophyte species was exclusive to polluted sections. The distribution of several plant species across the rivers and the river banks was noted and it appears that although tolerant species can grow partly submerged in clean water, they avoid direct contact with polluted water.