Fast and sensitive techniques are needed to determine microorganism presencein liquid samples. In this research, the feasibility of using light scattering spectrometry for enumeratingthe biological particles in liquid samples was investigated. A particle size spectrometer was used tocount six commonly found microbial species suspended in liquid with and without microbiological stainsapplied: Pseudomonas fluorescens, Micrococcus spp. vegetative cells and Bacillus subtilis var. nigerendospores were stained with Acridine Orange and Crystal Violet, while Cladosporium cladosporioides,Penicillium melinii and Aspergillus versicolor fungi were stained with Acridine Orange and LactophenolCotton Blue. The counts obtained with the spectrometer were compared with those obtained with a phase-contrastmicroscope. It was found that the spectrometer counted about 32 % of non-stained B. subtilis endosporesand this percentage increased to almost 90 % for stained endospores. Among the investigated species offungi, the counting efficiency of P. melinii was the only one significantly affected by the applicationof the stain Lactophenol Cotton Blue: the fraction of counted fungal spores increased from 64 % (non-stainedspores) to about 100 % (stained spores). The observed difference in counting efficiency may serve asa basis for differentiating biological from non-biological particles in liquid samples.