Mother rediae of Echinostoma revolutum were transplanted individually from infected to uninfected Stagnicola palustris ranging in length from 10 to 25 mm. The number of daughter rediae present at 30, 40, and 50 days after transfer was related to the size of the snail, progressively so with the age of the infection. There was also a direct relationship between the size of the snail and the maximum size attained by the rediae which in general was greater than that seen in naturally infected snails. When daughter rediae were transplanted in groups of five into snails 20 to 23 mm long, and examined 40 or 60 days later, none of the snails contained additional rediae and none of the rediae contained rediae and cercariae together, as is occasionally seen in natural infections. Transplantation of trematode larvae from infected to uninfected snails has been useful in studies on various aspects of larval development. Donges (1963) artificially infected snails with individual rediae of an echinostome (Isthmiophora spiculator) to determine the exact sequence of larval stages. Chernin (1966, 1967) developed a technique for transferring Schistosoma mansoni sporocysts and echinostome rediae into the cephalopedal sinus of Australorbis glabratus, and Heyneman (1966) successfully established echinostome infections by introducing either miracidia or rediae into the tissues of Lymnaea rubiginosa and Indoplanorbis exustus. Changes in the transplanted rediae and metacercariae of Metagonimoides oregonensis were studied by Meade and Pratt (1966). In a previous study the relationship between the size of the host and the size of the redial population of Echinostoma revolutum was examined (Zischke, 1967). Although it was established that the number of mother rediae produced by the sporocyst was related to the size of the snail host, the number of daughter rediae generated by mother rediae could only be indirectly estimated. Moreover, in laboratory Received for publication 23 June 1967. *Present address: Department of Biology, St. Olaf College, Northfield, Minnesota. From a dissertation submitted to the Graduate School of Tulane University in partial fulfillment of the requirements for the degree of Doctor of Philosophy. The study was supported in part by Grant 5-TI-AI-02 from the NIH, U. S. Public Health Service. infections only two generations of rediae were found whereas in naturally infected snails a third generation of rediae occasionally was seen in daughter rediae that were producing cercariae. The present study was undertaken to determine the number of daughter rediae produced when mother rediae of E. revolutum are transplanted into uninfected snails (Stagnicola palustris) of various sizes, and to establish whether transplanted daughter rediae produce an additional generation of rediae. MATERIALS AND METHODS Mature mother rediae, obtained from snails (S. palustris) which were previously exposed to 10 miracidia, were introduced through holes, drilled with a 20-gauge hypodermic needle, into the region of the body whorl of laboratory-reared snails. Mature daughter rediae were similarly introduced, either between the body whorl and the first whorl of the spire or between the first and second whorls of the spire. The rediae, together with a small amount of balanced salt solution (Dulbecco and Vogt, 1954) were placed in the hemocoel by means of a capillary tube (inner diameter, 0.4 to 0.8 mm). In vitro survival tests proved the balanced salt solution to be a suitable medium for maintaining rediae outside of the host. Care was taken to rupture the mantle so that the larvae would not be entrapped between the mantle and the shell. The opening was sealed with silicone and over-sealed with melted paraffin applied with a small artist's brush. The wound healed completely in 4 to 6 days. All procedures were carried out under a hood and with sterile instruments and glassware. Reports published by several authors subsequent to the completion of this study suggest that the latter precautions may not greatly increase transplant success. All snails were maintained at 20 to 25 C.