The freshwater wetland plants, Echinochloa crusgalli crusgalli and Echinochloa crusgalli zelayensis, and the saltmarsh plant Spartina alterniflora were exposed to the herbicides metolachlor and norflurazon in two types of toxicity tests: (1) seed germination and early seedling growth in water, and (2) seedling survival and growth in natural and synthetic sediments. The synthetic sediments were formulated to be similar to the natural sediments with regard to particle size distribution and organic content. The herbicides did not affect rate of germination, but significantly inhibited rate of early growth and survival and rate of growth of older seedlings in sediments. Echinochloa was more sensitive than Spartina to both herbicides. Inhibition of the growth rates of the two varieties of E. crusgalli was similar in natural and synthetic sediments, but inhibition of growth of S. alterniflora was greater in synthetic than in natural sediment. It is concluded that the species tested may be used for estimation of potential effects of toxicants on wetland plants and that synthetic sediments of known composition may be used in sediment toxicity tests.