Arrangement for collecting and composting night-soil and other biodegradable material, comprising a space (23) in which a starting bed (29) has been spread out in order to accelerate the breakdown of material (30) which is dropped down onto the starting bed. The space (23) is lined with an inner layer of a flexible, liquid-impermeable material forming a sack-like container (8) which is open at the top. The starting bed (29) is arranged in the sack-like container, and the arrangement comprises members (40) for pressing the upper edge portion of the container against a ceiling (36) which is located under the floor (32) of at least one overhead toilet and is provided with at least one opening for attachment to a toilet bowl. The sack-like container (8) is designed to enclose the material (30) while the latter is being broken down, and the container comprises members for conveying liquid, which has been separated off from the material, via a screen arrangement (31). The sack-like container (8) is also designed to serve as an enclosure for transporting the completely or partially broken down material. The invention also relates to a method for collecting and composting night-soil.