in vitro study on the impact of fish sera on the survival and fine structure of the eelpathogenic acanthocephalan paratenuisentis ambiguus the effects of fish sera on the growth and fine structure of infective larvae of the eelpathogenic acanthocephalan paratenuisentis ambiguus eoacanthocephala tenuisentidae were studied under in vitro conditions using sera from the final host anguilla anguilla and from two accidental fish hosts as well as fetal calf serum as controls larvae were also kept in medium in the absence of serum and in experimentally infected eels sera from the accidental fish hosts carp and rainbow trout exerted toxic effects on the acanthocephalans worms maintained in medium containing sera from these two fish were contracted and displayed inverted probosces moreover the tegument exhibited vacuolization and the formation of necrotic areas including lysis of the mitochondria due to these effects the parasites died at 21 rainbow trout or 2150 days carp postincubation eel sera had no toxic effect on the infective larvae the growth of the larvae in medium depended on the composition of the latter but was reduced as compared with that in the natural final host based on these results we conclude that components of the hosts blood sera play a role in the determination of the host specificity of p ambiguus