Most horsehair worms are observed in their adult, free-living stage among the vegetation near edges of ponds and streams, although a few semiterrestrial species occur in damp soil. A small group of marine species, parasitic on crustaceans, have been collected from coastal environments. Color usually ranges between light tan to dark brown, although yellowish and black individuals have been observed. Size and coloration vary considerably, even within the same species. Males are smaller than females, and the two sexes may be found in equal and unequal numbers in the same population.
Horsehair worms reproduce sexually, in spring, early summer, or autumn.
Eggs, often numbering in the millions, are laid in long gelatinous strings. For aquatic forms, the incubation period may range between 15 and 80 days, depending upon water temperature. The mode of development of nematomorphs after hatching has been the subject of much debate. Some experts suggest that the larvae encyst on vegetation or other substrates along the water's edge soon after hatching. Eventually, some of these cysts are ingested by hosts feeding on these substrates.