The significance of <i>Diptostomum spathaceum</i> to trout farming was highlighted in a nationwide survey and the importance of the snail intermediate host <i>Lymnaea peregra</i> was emphasised by the fact that fish kept in earth units, in which snails were more prevalent, were more susceptible to infection. Studies revealed that <i>Diplostomum</i> spp. metacercariae, present in both the lenses and humours of eyes, were abundant in September. Differences in metacercarial numbers between fry and juvenile fish, and between years, were attributed to the absence of snails within the fry units and differences in snail numbers respectively. Molluscicides were ineffective in eradicating snails but clearing vegetation from units reduced numbers of snails and infection levels dramatically. Diplostomiasis appeared to be more prevalent in areas of a farm where water flow was reduced and where snails were abundant. Comparison of the internal defence system of two populations of <i>L. stagnalis</i> indicated that in both, haemocyte numbers increased and the relative proportions of haemocyte types changed with snail age. Haemocyte phagocytic activity also increased with snail age. However, one population comprised mainly Type I snails (Haemolymph agglutinates erythrocytes at high titres) whilst the other comprised mainly Type II snails (low agglutinin titres). Haemolymph from Type I and II juveniles had comparable agglutinating activities but as snails grew, activity increased in Type I snails whilst that in Type II snails remained low. Plasma from Type I and II snails increased the phagocytic activity, Type I plasma having a more pronounced effect. During the course of the infection with <i>D. spathaceum</i>, both haemocyte- and plasma-associated activity decreased in Type I and II snails. Most markedly, 24 h post-infection (p.i.) phagocytic activity was suppressed and plasma taken from snails 24 h p.i. had a suppressing effect on haemocytes from uninfected adults. An <i>in vitro</i> bacterial killing assay demonstrated that <i>L. stagnalis</i> haemocytes recognize and effectively kill the bacterium <i>Aeromonas salmonicida</i>. Cytochalasin B decreased bactericidal activity implicating both intracellular and extracellular cytotoxicity of haemocytes.