The term Cercaria is frequently used in a generic sense, in combination with specific names, to designate caudate larvae of the digenetic trematodes. Although the study of life cycles has received much attention in recent years, the adults and larval stages of relatively few species have been experimentally correlated. Hence it is convenient to describe larval trematodes as species of Cercaria although this term is obviously suprageneric in rank. In studying cercariae, the differentiation of species has been based almost entirely on morphological characters. It is frequently difficult to detect morphological differences in closely related species. However, marked differences in the behavior of such larvae may be noted in certain groups and a study of their swimming habits and reactions to various stimuli may be the deciding factor in differentiating species. The present study deals with the behavior of four species of cercariae belonging to a restricted group, the Pleurolophocerca Group (Sewell, 1922), and emphasizes the value of careful physiological observations in differentiating members of this group. Many investigators have noted phototactic responses and swimming habits of cercariae but, until recently, have recorded only brief notes and references to their behavior. Cort and Talbot (1936) utilized swimming habits and resting positions in studying four species of schistosome cercariae, Cercaria elvae Miller, 1923; C. physellae Talbot, 1936; C. stagnicolae Talbot, 1936; and C. douthitti Cort, 1914. Although the first three of these are very similar morphologically, Cort and Talbot observed that careful study of their swimming habits made it possible to identify them with a low-power lens. They also concluded that C. elvae and C. ocellata La Val., 1855, are not synonymous as suggested by several authors, since differences in the position of body and tail while resting could be correlated with differences in body size. Miller (1926) studied the swimming behavior of five species of larval trematodes infecting the marine snail, Cerithium litteratum Born. He found their behaviors so characteristic that each could be identified by inspection with the unaided eye. Of this group, the pleurolophocercous form, Cercaria P Miller, 1926 (= C. floridensis McCoy, 1929), is very similar in behavior to two of the species included in the present study. Received for publication, February 2, 1939. 343