Traditionally, the abundance, cell size distribution and species composition of phytoplankton is determined by microscopic counts. In recent years, various express methods have been developed for phytoplankton routine studies. However, each of these methods has drawbacks. It is important for aquatic ecologists to understand the advantages, disadvantages and limitations of these methods. We compared the sensitivity of three express methods (multichannel fluorimeter FluoroProbe, imaging flow cytometer FlowCam, CASY particle counter) to changes in cell concentration of three phytoplankton species (Chlorella vulgaris, Arthrospira platensis Gomont, and Nostoc sp.). We also assessed the possibility of express methods to estimate the cell abundance of different species in the mixed samples. All instruments showed a high sensitivity to changes in the cell abundance of different phytoplankton species and a mixture of these species. Either method, once calibrated, can be reliably used to estimate the abundance of a single-species/laboratory culture of microalgae. At the same time, FlowCam without preliminary calibration recorded the cell abundance closest to microscopic counts. When analysing a mixture of three microalgae differing in their cells size and spectral characteristics, FluoroProbe showed the highest accuracy when assessing the ratio of species in the mixture, and FlowCam - when assessing abundance. To study mixtures of algae and/or natural phytoplankton communities, it is advisable to use jointly a flow cytometer and a multichannel fluorimeter. The images of algae saved by the flow cytometer, if necessary, will allow them to be determined with a certain accuracy to the species. Information about cells size and spectral characteristics, obtained by two methods, will be detailed enough for such common tasks as studying trophic interactions between phyto- and zooplankton or creating warning systems about unwanted mass development of phytoplankton and its individual groups (for example, cyanobacteria).