1. Euglenamorpha hegneri is the name proposed for a new Euglena-like flagellate found in the rectum of tadpoles and first reported by Hegner ('22). 2. Most of the tadpoles examined were those of the bullfrog obtained from the pond in the Botanical Gardens of the University of Pennsylvania. Some tadpoles of Rana palustris collected by Dr. C. L. Parmenter and fed on vegetation from the same pond were also examined. About 60 per cent. of the 117 bullfrog tadpoles examined in 1922 and 16 per cent. of the 30 examined in 1923 harbored Euglenamorpha. About 60 per cent. of the 9 tadpoles of Rana palustris examined in 1922 and also of the 35 examined in 1923 were infested with this flagellate. Eight tadpoles of R. clamitans(?) and 8 of Hyla versicolor(?) examined at Woods Hole in August, 1923, all contained Euglenamorpha. 3. There are two varieties: one, green, is the type, and the other, pellucida, is a colorless derivative. Typical individuals of the two varieties are so different that without intermediate stages they would be put into different species or into different genera. 4. The green (type) variety has a cylindrical or cigar-shaped body, chlorophyll, stigma, paramylon granules, compact nucleus, three flagella with swellings on their roots near the stigma, surface striæ passing from the left over to the right. It may occasionally divide by mitosis within the host. 5. Variety pellucida has an elongately conical body, little or no chlorophyll, no stigma, no swellings on the usually four or six flagella; has enlarged reservoir, nucleus expanded, and surface striæ usually passing from right over to the left or longitudinal. Amitosis as well as mitosis occurs frequently in the host. 6. Transitions between the two varieties are found and it is believed that the green variety transforms into the colorless one. Division of the green variety in the host is rare, but appears to be accompanied by transformation which may also take place without division. Once established, the colorless variety may multiply rapidly by both mitosis and amitosis and consequently outnumber the green one. 7. Attempts to culture the flagellates outside the host were more successful with hanging drops than with dishes. The most successful culture was a hanging drop of 0.6 per cent. NaCl in which the green variety multiplied for three weeks, then gradually declined, but some few were still alive after five months. In other cultures a few appeared to transform from the green to the colorless variety. 8. The movements of E. hegneri, both swimming and metabolic, may be very rapid. In some cases the organisms appear to attach themselves to the wall of the rectum or other object by one flagellum to vibrate the others near the body, thus possibly serving both a respiratory and a nutritive function. They are positively phototropic. 9. Outside the host (in culture) the green variety may assume a resting state approximating encystment in which it may divide and by means of which, presumably, it reaches a new host. The colorless variety is not known to assume the resting state and appears to be too unstable to be a permanent form. 10. One is tempted to regard the entire situation as revealing an uncompleted series of stages in the evolution of a colorless parasite from a free-living, green euglenoid flagellate, but the evidence is not entirely conclusive.