Two species of earthworms, Lumbricus terrestris and Eisenia fetida, were used to de-clog sand filters purifying domestic sewage. The experimental set-up consisted of 6 filtration columns (4 research filters and 2 control filters) filled with fine sand (d₆₀=0.2 mm, d₁₀=0.1 mm). The sewage parameters at inflow to the filters were typical of the septic tank effluent. The filters were fed about 5.5 dm³/day at average doses of 31 cm/d. The TSS cumulative loading after three months was equal to 931.9 g/m². At the start of filter operating the average outflow of filters was equal to 105.6 cm³/min; at the end (except for filter Nos. 2 and 6) it was 0.3 cm³/min per filter and the clogging was achieved. A significant increase in outflow rates was observed two weeks after the application of worms, and it was equal to 94.7±2.9 cm3/min on average (average value for control filters: 7.1±0.8). The content of organic matter in the clogging layer of filters 4 and 5 was equal to 9.6±0.5 mg of dry mass per 1 g of sand, on average, and was significantly lower than the content of organic matter in the clogging layer of control filters (28.2±1.0 mg dry mass per 1 g of sand). There was no difference between control and research filter treatment efficiency (COD and NH_4-N). Four months after the worm application, the increase in average outflow from research filters was still observed, thus the de-clogging effect had been observed as long as the worms were alive.