A relatively rapid and sensitive method for the bioassay of seawater for dissolved biotin is described. The uptake of14CO2by cells ofAmphidinium carteriin 2 h following a 94-h preincubation under carefully controlled conditions is proportional to biotin concentrations in seawater. Cell numbers can also be used to measure response to vitamin concentrations but the incubation time is extended to 144 or 168 h. The range of biotin concentrations capable of being assayed is 0.2–6.0 ng/l. These amounts are considerably less than those previously described.