Temporal changes in diversity and similarity of a phytoplankton community were investigated in relation to external hydrological disturbance in the Ben Chifley reservoir from September 1998 to January 2002. Species richness varied by a factor of 4-5 at each of three sites studied during the period (n = 53 at each site). Species diversity (measured using Simpson's D and Shannon-Wiener's H, based on primarily genus or species number and cell densities) varied by a factor of 8-10, whereas similarity between two consecutive sampling dates (measured using Hurlbert's index and Pinkham and Pearson's B) varied by a factor of 10-46. When diversity was measured with H, it had an approximate quadratic (convex) relationship with similarity, as measured with Hurlbert's index. However, diversity was seldom related to external hydrological disturbance (measured as intensity and variability of daily inflow rates between two consecutive sampling dates). Similarity was significantly and negatively related to disturbance variability. These results suggest that the mechanisms that regulate diversity and similarity may differ from each other, and question the usefulness of observed approximate quadratic relationships between similarity and diversity indices when assessing the effect of disturbance on diversity. Such relationships may therefore not provide support for Connell's (1978) intermediate disturbance hypothesis.