A comparative kinetic study of nitrate-limited growth and nitrate uptake was carried out in chemostat cultures of Ankistrodesmus falcatus, Asterionella formosa, Fragilaria crotonensis. In each species growth rate (microgram) was related to total cell nitrogen or cell quota (q) by the empirical Droop growth functions. Nitrate uptake was a function of both external N concentration and q. The apparent maximum uptake rate (v sub m) at a given microgram was inversely related to q - q sub o, where q sub o is the minimum quota. The apparent half-saturation constant for uptake (k sub m) appears to show a slight inverse trend with microgram, although statistical analysis show that this trend is inconclusive.