AbstractToxic inhibition of three communities of filamentous microorganisms by chlorine and hydrogen peroxide was investigated in relation to the total suspended solids concentration of bulking acitvated sludges. The filament communities studied were dominated by type 1701,Sphaerotilus natans, and the combination of type 021N andThiothrixspp., respectively. Activity of filamentous and gross biomass was assessed separately using the malachite green‐INT (MINT) test. The effectiveness of toxicants was characterized in terms of the concentration required to inhibit 50% of microbial dehydrogenase activity (IC50). IC50s were found to be proportional to total suspended solids concentration. Values for filamentous biomass were generally lower than those for gross biomass. Relative sensitivity (ratio of gross IC50to filamentous IC50) did not vary significantly over the practical range of total suspended solids concentraton. This parameter was unaffected by the nature of the toxicant inS. natansand type 021N/Thiothrixdominated sludges, but was toxicant specific in type 1701 dominated sludge. Overall,S. natansand type 1701 were the least sensitive of the filament types studied. Bulking caused by these organisms has been successfully cured by toxicant addition in numerous past instances. It is therefore expected that type 021N,Thiothrix, and most other types of filamentous bulking can also be controlled by this techniqu