A water-soluble fraction of green leaves of Zostera marina L. decreased carbon uptake rates in diatoms found as epiphytes at Roberts Bank (Lat 49°2′N; Long. 123°8′W) on the west coast of Canada. Extract concentrations of 11 to 1 μg dry leaf ml-1, added immediately before 14C-uptake was measured over a 3-h period, reduced uptake to 36 to 80% of controls, respectively. A lower concentration (0.6 μg dry leaf ml-1) reduced uptake significantly in only one of four cultures. Addition of the whole extract 3 d before carbon uptake was measured or in one-third doses, 0, 1, and 2 d before, resulted in inhibition similar to that observed when the whole extract was added immediately before the 14C.