The line-intercept method was adopted for quantification of aquatic filamentous microorganisms. The cumulative length of filaments in a sample is calculated from the number of intercepts between filaments and test bars of known length. The product of the cumulative length and of filament diameter reveals the total biovolume of filaments. The method is suitable for reliable and statistically correct quantification of long or dense filaments immeasurable individually. The combination of epifluorescent microscopy and image analysis speeds up sample processing and supports differentiation between heterotrophic and autotrophic filaments. Parallel testing of the line-intercept method in Utermohl sedimentation chambers revealed tight correlation (r(2) = 0.89) of both methods for cyanobacterial filaments. (C) 2001 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.