Grass shrimp (Palaemonetes pugio) were exposed to either waterborne or sediment-source concentrations of fenvalerate, cypermethrin, 1,2,4-trichlorobenzene (TCB), tributyltin oxide (TBTO), triphenyltin oxide, and di-n-butylphthalate in static or flow-through test systems. Similarly, amphioxus (Branchiostoma caribaeum) were tested with fenvalerate, TCB, and TBTO. The LC50 and no-effect and 100% mortality concentrations are reported from 96-hr and 10-day tests. The toxicity of contaminated sediments could be explained by chemical partitioning into overlying or interstitial water. Amphioxus is not recommended as a routine test species because of (1) difficulty in distinguishing severely affected from dead individuals, (2) inability to determine the status of burrowed animals without disrupting sediment, (3) their relative lack of sensitivity in acute exposures to toxic chemicals, and (4) difficulty in routine collection of sufficient numbers of animals. Grass shrimp, however, are useful as an epibenthic test species for waterborne and sediment-source toxicants.