Zooparasitic nematodes are derived from soil nematodes and parasitism apparently originated only after land animals appeared. A major line from soil dwelling rhabditids gave rise to the secernentean parasites, which constitute about 98% of all genera in vertebrates and about 80% of those in invertebrates. A minor line evolved from the predominantly soil-dwelling dorylaimids and gave rise to adenophorean parasites, including mermithoids, trichinelloids, and dioctophymatoids. Nematodes are common in terrestrial invertebrates but rare in aquatic invertebrates (e.g., polychaetes, molluscs, crustaceans). The fauna in terrestrial vertebrates is rich and diversified. In contrast, the fish nematode fauna is limited. Only 17 families of nematodes occur in fishes and only 5 are unique to the fishes. No unique nematode superfamilies occur in fishes. The fish nematode fauna seems closely related to that in terrestrial vertebrates and is evidently derived from it since almost three quarters of the families are shared with those in terrestrial vertebrates. Transfer to fishes from terrestrial hosts during the course of evolution probably occurred mainly through heteroxeny and paratenesis; however, a few monoxenous forms (oxyuroids) from terrestrial arthropods became established in shallow water fishes. Only a few nematodes (i.e., mermithoids) transferred from terrestrial to aquatic invertebrates with the result that nematodes are extremely rare in freshwater and marine invertebrates. The nematode fauna of marine mammals is limited mainly to heteroxenous forms with affinities to nematodes of terrestrial mammals.