A comparative survey on the capacity of several nitrogenous compounds to support marine phytoplankton growth showed urea as the best and D-glucosamine as the poorest supplier of organic-nitrogen at a concentration of 500 /g-at. N per litre (Antia et al., 1975a). The quality and degree of growth on glucosamine was always so inferior that the evidence appeared insufficient to establish whether this compound was actually being metabolized by the algae. By analogy with the substrate-utilization kinetics reported for growth of other micro-organisms such as yeast (e.g. Tseng & Wayman, 1975), the probability was considered that the glucosamine concentration used in the above survey may have been too low for adequate uptake under the test conditions chosen and that elevated concentrations, in facilitating such uptake, may provide unequivocal evidence of algal capacity for metabolism and utilization of this substrate. However, such concentration elevation would require to be below levels that may become inhibitory to growth, and these inhibitory levels may themselves vary from one species to another. Antia & Chorney (1968) reported 5 mM glucosamine to be toxic to growth of the cryptomonad Hemiselmis virescens, whereas McLachlan & Craigie (1966) observed growth of the diatoms Cyclotella cryptica and Thalassiosira fluviatilis on five times this level of concentration.