Myxidium biliare sp. n., a new myxosporean species parasitizing the gall bladder of Galaxias maculatus (Jenyns), in Patagonia, is described. Its coelozoic plasmodia were floating free in the bile. Spores are fusiform 13.7 ± 0.9 µm long and 6.9 ± 0.6 µm wide, with rounded ends in frontal view and slightly pointed ends in sutural view; shell with ridges and sinuous sutural line. Both maximum prevalence and maximum percentage of immature plasmodia occurred in summer. In winter the prevalence and the percentage of immature plasmodia fell to their lowest values. Prevalence was independent of host sex but increased with host length. Prevalence in 15 Patagonian Andean lakes (situated from 39°25'S to 41°30'S) ranged between 4.2% and 70%.
Posterotestes gen. n. (Digenea: Apocreadiidae, Apocreadiinae) is proposed for specimens with the following features: spiny body, posterior position of gonads, extension of vitelline follicles up to level of intestinal bifurcation and absent at post-testicular space, long oesophagus and extension of caeca up to anterior testis. Posterotestes unelen sp. n. is described from the intestine of the native fish, Percichthys trucha (Cuvier et Valenciennes) (Osteichthyes: Percichthyidae) from Patagonian Andean lakes.