Most aquatic oligochaetes burrow slowly in the sediment of lakes and rivers, ingesting and digesting large amounts of silt and sand. Bacteria and other microorganisms are digested selectively. The internal water skeleton (coelomic fluid), the muscle layers of body wall, and chaetae, anchoring the body to the surrounding sediment, are their main tools for locomotion. Many oligochaetes are tolerant of oxygen deficiency. The Family Tubificidae is the most common and diverse group, while many Lumbriculidae and some other groups prefer cooler, among them subterranean habitats. Some species of terrestrial "earthworms" in the broad sense (Family Lumbricidae, etc.), as well as of small terrestrial Enchytraeidae, occur also more or less regularly on the bottom of fresh waters.

A derived group, the Family Naididae, is adapted to life on bottom surface and macrovegetation and have developed the ability to swim and, in some genera, even simple eyes. Many of them feed on algae; one genus (Chaetogaster) is in transition to predatory feeding.

The Order Branchiobdellida is highly adapted to commensal life on crayfish. Their body is leech-like, devoid of any chaetae but equipped with sucker-like adhesive organs on both ends, as well as with a pair of chitinized jaws. The branchiobdellidans feed on small animals but can also consume the host's tissues, then being true parasites.

All oligochaetes can reproduce in a sexual way. Their individuals are hermaphroditic. After copulation, the partner's sperm is stored in the spermathecae or, seldom, in the external spermatophores. Eggs are laid in cocoons excreted by a special epidermal organ, the clitellum, and are fertilized there with alien spermatozoa introduced from the spermathecae. There is no larval stage of development. Some genera and species of different families have invented independently an asexual way of reproduction by architomy (fragmentation with subsequent regeneration). The Naididae are capable of paratomy (division at special budding zones between the zooids, where regeneration has already begun), while sexual reproduction has become rare.
Aeolosomatida, the most common representatives of freshwater polychaetes, are convergently similar to Naididae, considering both their way of life on water plants and mainly asexual reproduction by paratomy.