Determining the fraction of viable cells in algal cultures is critical to improve the understanding and control of algal microbiology, ecology, and biotechnology. Whereas current techniques for algal viability determination can be rather cumbersome, this paper describes a new assay that enables the rapid quantification of algal viability using only spectrophotometric measurements. This technique, henceforth named CAVA test, relies on the selective adsorption of erythrosine by non-viable cells and was validated on the algal species Chlamydomonas reinhardtii and Chlorella vulgaris. The results obtained by the CAVA test were in good agreement with the in situ measurement of oxygen production rates. In addition, the CAVA test was shown to quantify the viability of algal samples regardless of the cause of death (heating, UV-irradiation or H2O2 exposure). The CAVA technique has28 therefore the potential to offer fast and universal approach to measure the viability of algal samples.