Macroinvertebrate assemblages, physico-chemical factors, epilithic algal biomass and leaf decomposition were examined in streams draining the Charming Creek and Stockton-Millerton coalfields north of Westport, South Island. Three invertebrate community types were evident in the streams depending on the degree of contamination by acid mine drainage. A diverse (9-31 taxa) "clean water" fauna dominated by the mayfly Deleatidium, larval Elmidae and various Trichoptera and Tipulidae occurred down to pH 4.5 where iron concentration was < 0.69 mg l⁻¹ and aluminium < 0.41 mg l⁻¹. At sites subject to moderate acid mine drainage and iron hydroxide precipitation, diversity was lower (9-13 taxa) and invertebrate density was often very low. Spaniocercoides phi/potti, a Zelandobius species and Oxyethira albiceps dominated. In the most polluted streams, where pH < 3, conductivity> 300-450 µS cm⁻¹ and metal concentrations were elevated (Fe> 5.7 mgl⁻¹, Al > 3.4 mg l⁻¹ ) taxonomic diversity was very low (2-6 taxa) but invertebrate abundance was often very high. Chironomidae made up 80-100% ofindividuals caught. Acid streams were dominated by habitat generalists rather than specialist acidophiles. Biomass ofperiphytic algae examined experimentally in 7 streams (pH 2.8-6.2) was greatest at low pH. It appeared to be affected by precipitation of iron hydroxide, nutrient concentration and the absence ofbicarbonate below pH 5.5. Grazing by invertebrates did not appear to have a significant effect on algal biomass. Decomposition ofbeech (Nothofagus) and kamahi (Weinmannia) leaves in 6 acidic-eircumneutral streams lacking shredders was slow, and differences in decomposition rates in the contrasting streams were evident only after 6 months. As in other parts of the world, acid mine drainage has a severe effect on New Zealand stream ecology.