Single-species, steady-state chemostat cultures of two freshwater diatoms showed that Fragilaria crotonensis had a lower equilibrial requirement for silicate than did Tabellaria fenestratra. Using this information, resource competition theory predicts that Fragilaria should be able to invade silicate-limited equilibrium populations of Tabellaria, but Tabellaria should not be able to invade silicatelimited Fragilaria. This prediction was supported by a series of invasion experiments. The two species did not have detectable differences in their phosphate requirements. Invasion experiments showed that Fragilaria could invade Tabellaria cultures, but that Tabellaria could not invade Fragilaria cultures under phosphate-limited conditions. These results are consistent with predictions based on previous studies of the phosphate physiology of these genera.