The test sensitivities and reproducibilities based on different response endpoints (cell density and total cell volume) were compared using both bath and continuous algal toxicity tests. Parameters related to algal cell density are found to be more sensitive and reproducible than total cell volume which bears a direct correlation to algal biomass or dry weight. The magnitude of differences in the median effective concentration values could be upto six folds. The main cause for the above discrepancies is because the cell density and total cell volume are two response endpoints reflecting the degree of inhibition on the binary fission mechanism and overall photosynthesis, respectively. At low toxicant concentrations, the inhibitory effect is primarily on the cell division rather than on the cellular photosynthesis. Thus, more severe toxic effects are expected to be observed based on the cell density than the total cell volume. The results of this study clearly demonstrate that sensitivity and reproducibility can be significantly improved if response parameters are calculated based on the cell density. Traditionally, cell density data are often converted into biomass before assessing the effects of toxicants. This procedure may not be necessary and may reduce the test sensitivity.