Polymorphism has been defined by Ford (1953, 1956) as ~the occurrence together in the same habitat at the same time of two or more discontinuous forms of a species, the rarest of which is too frequent to be maintained merely by recurrent mutation'. There has been little attempt to apply this definition to nematodes of ruminants. In the sub-family Ostertagiinae, the first indication of polymorphism emerges from data presented by Copland (1965) who, whilst attempting to establish a pure culture of Ostertagia circumcincta by male selection with unknown virgin females of mixed Ostertagia spp. found males of O. circumcincta, O. trifurcata and Teladorsagia davtiani for several generations before finally achieving a pure culture. These findings were reported by the author as cross-fertilization between species even though hybrids were not in evidence. That hybridization can occur between two apparently closely related species has been shown by Isenstein (1971b) in the subfamily Cooperiinae (Cooperia oncophora and C. pectinata) although this only occurred under closely controlled experimental conditions where the nematodes were presented with no other choice of mate. Furthermore, Isenstein (1971a) concluded that C. oncophora is polymorphic with C. surnabada (= C. mcmasteri) as an alternative morph. In yet another sub-family, the Haemonchinae, it has been shown that the relative frequency of smooth, knobbed or linguiform flap types of female Haemonchus contortus demonstrates a polymorphic relationship (Daskalov, 1971; LeJambre, 1977). Thus evidence has been presented of polymorphism in three sub-families of the family Trichostrongylidae. Consequently, a series of observations based on the hypothesis that O. ostertagi is a polymorphic species has been performed at this laboratory over the past few years. Sufficient evidence has now been accumulated to state categorically that O. ostertagi is polymorphic with O. lyrata as an alternative morph (Michel, Lancaster & Hong, in preparation). While this work was in progress, Daskalov (1975, Helm. Abst. Ser. A, 44, 5298) presented evidence that O. circumcincta is a polymorphic species with O. trifurcata and T. davtiani as alternative morphs. Recent work at this laboratory has confirmed the findings of Daskalov with regard to O. circumcincta and O. trifurcata (Michel, Lancaster & Hong, 1979, unpublished observations) but the present authors have thought for some time that T. davtiani is merely a host-induced change in O. trijurcata of a similar nature to that' which causes changes in the morphology of the flap of female O. ostertagi (Michel, 1967). This now well recognized phenomenon is not confined to O. ostertagi, as both Lancaster (1968) and Denham (1969) saw similar changes in O. circumcincta females which have since been confirmed as a host-induced effect (Waller & Thomas, 1978). Lancaster & Hong (1971), after observing 'flaplessness' in yet another species, O. leptospicularis (= O. crimensis), suggested that, in all probability, this phenomenon would eventually be found in all females of the sub-family Ostertagiinae which possessed flaps. It seems not unreasonable to suppose, therefore, that the host may similarly affect comparable areas in the male ostertagian. In the case of O. trifurcata this could well be SjOberg's organ (as defined by Dr6~d~, 1965) which