Rana temporaria tadpoles which had been naturally infected by larvae of Polystoma integerrimum were kept in the laboratory under conditions precluding the continuous re-infection which occurs in nature. One tadpole was seen to have two larvae attached to the skin of the ventral surface of the body, which were moving towards the posterior end. The tadpole measured 7·9 mm. in body length and 20·8 mm. in total length, and its stage of development corresponded to stage 5 in the table of development and metamorphosis compiled by Rugh (1951); it was collected in mid-May and had been kept for twelve days under laboratory conditions. The tadpole was fixed and dissected: the bladder had not developed, and the gills showed no indication of atrophy; the alimentary canal was long and heavily coiled, and filled with food. In addition to the two larvae observed on the skin, one larva was found in the branchial chamber, two were in the cloacal tube, seven occupied the cloacal chamber and two had travelled up the right ureter to the kidney; the alimentary canal was free of larvae. All fourteen larvae were gyrodactyloids.