SUMMARY1 We used biovolume as a simple non‐destructive means for estimating biomass for several tubificid and chironomid species. Biovolumes were determined by treating the organisms as cylinders and measuring body lengths and diameters; estimates were verified by measuring the displacement of water by live organisms in a 5‐ml burette. In addition, factors were determined for the conversion of biovolume (V) into wet weight (WW), dry weight (DW) and ash‐free dry weight (AFDW).2 For six tubificid species, regression functions between the diameter of segment XI (D11) and the biovolume enabled the prediction of any biovolume merely by measuringD11. This method, however, underestimated biovolumes ofBranchiura sowerbyiby 38–47% compared with burette measurements. This difference is largely attributable to the mountant we used during identification.3 For ten chironomid species a linear relation was found between body length and the diameter of abdominal segment 5. The biovolume ofChironomus muratensiswas underestimated by an average of 12% compared with burette measurements.4 Specific gravity (WW:V) amounted to 1.07 and 1.05, DW:WW ratios were 0.20 and 0.142 and AFDW:WW ratios were 0.182 and 0.129 for Oligochaeta and Chironomidae, respectively.5 The biovolume method is not yet very accurate for Oligochaeta, but to our knowledge, it is the only practical method that can be used in natural polyspecific communities that need to be treated with elucidating media for identification. For those Chironomidae that need no elucidation for identification, this method is time saving and just as accurate as the direct body sha