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SUMMARYMicroaerophilic organisms were monitored in sewage effluent undergoing two secondary treatments: air and oxygen-activated sludge. The mean numbers of Arcobacter cryaerophilus and thermophilic campylobacters detected in incoming sewage were 5639/100 ml and 1720/100 ml respectively.Secondary treatment in air tanks reduced the population of A. cryaerophilus by 971 % and of thermophilic campylobacters by 99-08 %, whereas treatment in oxygen tanks reduced the bacteria 97-8 % and 99-63 % respectively, showing that oxygen-activated sludge treatment was more effective. Subsequent tertiary treatment with 2 p.p.m. chlorine dioxide evidenced the removal ofA. cryaerophilus to 99 9 % and eliminated thermophilic campylobacters.Campylobacter jejuni and C. coli constituted 54-1 % and 45-9 % of 74 thermophilic campylobacter strains isolated. In air-activated sludge effluent C. jejuni was found more often, thus appearing more sensitive to oxygen.The most probable number assay used for detection of campylobacters, blood medium for enrichment and blood-free medium for plating, also appeared to be fit for A. cryaerophilus, the high density of which in urban sewage may be due to inflows from slaughterhouses.