Cell growth is an important parameter that requires accurate measurement during the cultivation of different microalgae and in biomass production for downstream processing. Also, the assessment of microalgal growth by optical density during process optimisation for wastewater treatment is sometimes confounded by suspended and/or particulate material and changes in medium turbidity. To address these challenges the current work sought to determine whether the one-step diphenylamine (DPA) method could be used reliably to quantify growth of the red alga, Galdieria sulphuraria and other microalgae. Results confirmed that the simplified DPA colorimetric method was sufficiently reproducible for the quantification of G. sulphuraria growth in phototrophic culture and that a linear relationship existed between an increase in cell culture dry biomass and the increase in cell density (OD₇₅₀) and DNA concentration (A₅₉₅). Similar results were obtained for laboratory cultures of locally sourced and isolated Chlorella sp., Scenedesmus sp. and the diatom, Phaeodactylum sp. For each isolate, the increase in algal dry biomass was linearly correlated with an increase in growth quantified either by monitoring OD₇₅₀ or DNA concentration using DPA at A₅₉₅. These findings confirm DPA as a simple and relatively inexpensive colorimetric method for the rapid determination of cell growth in microalgal cultures.