The work recorded in this paper deals with the transmission and life-history of certain trypanosomes and a trypanoplasm, inhabiting the blood of freshwater fish. It was undertaken at the suggestion of Prof. Minchin, to whom I am indebted for much kind advice. In 1905, Dr. Petrie (15) had observed that the goldfish in the pond at Queensberry Lodge, in the garden of the Lister Institute at Elstree, were almost invariably infected with a trypanosome. Three years later, Dr. J. D. Thomson (20) again found the flagellate in question, and gave an account of the appearances presented in cultures grown upon blood-agar. The trypanosome infection still persists in the blood of the fish in this pond, and, in addition, a trypanoplasm has appeared since 1908, when Dr. Thomson carried out his work.