SINCE the publication by the writer (Krull, 1932) of a paper in which it was stated that Cercaria bessiae Cort and Brooks, 1928, developed into Neascus ambloplitis Hughes, 1927, in a fish, Eupomotis gibbosus (Linnaeus), repeated experiments with this fish indicate that it may be affected by the parasite in different ways. These results are presented in an attempt to show what possible effects, direct or indirect, the parasite may have on fish in nature. The final host of this parasite is not known; feeding experiments carried out by Hunter and Hunter (1930) indicate that the parasite may be a species of Crassiphiala which matures in the kingfisher, Ceryle alcyon Linnaeus. The importance of the fluke and of any information concerning its life history or that of any closely related form is evident in view of the following observations. In 1931, the writer, while connected with the Institute for Fisheries Research, University of Michigan, had an opportunity to conduct an investigation of some brook trout ponds on the Sturgeon River, near Vanderbilt, Michigan. The ponds were formed in series by damming the river. Some of the changes resulting from this damming were widening and deepening of the river, inundation of much fallen timber, a gradual increase in depth along the shore, slackening of the current and increase in temperature of the water. These changes, apparently, increased the amount of food and improved conditions for survival of snails, of which there were countless numbers. The brook trout, Salvelinus fontinalis (Mitchill) in these ponds, both large and small, had black spots in the skin which on examination at Ann Arbor, Michigan, proved to be cysts containing a Neascus, tentatively identified as N. ambloplitis Hughes. Only a limited amount of material was available for examination, and since most of the cysts were degenerating at the time the infected fish were collected in June no final determination could be made. Incidentally it may be said that kingfishers, suspected of being the primary host, were numerous in the region where the trout were collected. On the basis of laboratory experiments with Cercaria bessiae, it may be assumed that under natural conditions the cercaria is responsible for the deaths of many small fish fry. Repeated infection experiments with fish, Eupomotis gibbosus, have been carried out for one year, and it has been found that there is more or less uniformity in the effects of the parasites on the host, the effects depending on the size of the fish. The fish may be divided roughly into 3 groups on this basis as follows: Small fish, 15 mm. to 30 mm. in length (tip of snout to base of caudal fin), when subjected to cercariae in large numbers succumb to the effects produced by them;