Silicic acid uptake kinetics for a field population of benthic diatoms were examined using a radioisotope tracer over a concentration range of 0-300 μmol L-1. The microphytobenthos half saturation constant (54 μmol L-1) and specific uptake rates (0.096 h-1) for silicic acid were well above those usually found in the pelagic environment. Silicic acid kinetics were hyperbolic only at low concentrations (< 60 μmol L-1). At higher concentrations, a second mechanism that did not suggest saturation was activated. Many benthic diatoms are motile and migrate vertically in the upper few centimeters of sediment where there are strong gradients of nutrient concentrations. The multiphasic uptake may allow them to take maximum advantage of the high silicic acid concentrations found at depth.