Black liquor and fine fiber produced by the pulp and paper industry are an environmental problem when disposed as an industrial waste. The major organic components, polysaccharides and lignin contained in these byproducts are potentially important in soil aggregation. Incubation studies were conducted to determine (i) the extent of soil aggregate formation due to black liquor (BL) and fine fiber (FF) as soil amendments, (ii) the relative importance of fungi and bacteria in soil aggregate formation through selective inhibition of fungi and bacteria, and (iii) whether soil dehydrogenase and â-glucosidase activities can be used as indicators of soil microbial activity that promotes soil aggregation when treated with biocides and BL or FF. Soil respiration rates peaked 2 days following the addition of BL or FF, but maximum wet stable macro-aggregation occurred after 20 days of incubation. Compared with BL or FF, fungicide treated BL or FF decreased wet stable macro-aggregates; but bactericide treated BL or FF increased wet stable macro-aggregates, suggesting that the fungal activity was likely responsible for the increases in wet stable macro-aggregates. Dehydrogenase and beta-glucosidase activities did not correlate with the soil respiration in the presence of biocides (fungicide, bactericide or both) treated BL or FF, suggesting that dehydrogenase and beta-glucosidase activities could not be used as indicators of soil microbial activity under these conditions. Additions of BL and FF at a rate of 1.5 g C kg-1 increased soil respiration, dehydrogenase and beta-glucosidase activities, and also improved the wet stability of macro-aggregates, suggesting that both byproducts could be a beneficial soil amendment.