Abstract: The method and apparatus for detecting the presence of an organism in a sample of liquid involves collecting an extinction spectrum of liquid sample, deconvoluting the spectrum to obtain a particle size distribution for the sample, comparing the spectrum and the particle size distribution with, respectively, a control spectrum and a control particle size distribution for the organism, and determining from the comparisons whether the organism to be detected is present in the sample. The apparatus and method permit on-line real-time quantitative classification, identification, and viability assessment of an organism such as a harmful microorganism in a water supply line.