The genus Renicola Cohn, 1904, was based on Monostoma pingue (Mehlis in Creplin, 1846) ; it is cosmopolitan in distribution and species have been found in the kidneys of birds belonging to various families. Cercaria rhodometopa , Perez, 1924, has a similar excretory system and the rhodometopous cercariae have been regarded as renicolid larvae, although attempts at experimental infections have so far proved futile. Cable (1963) reported that certain plagiorchid cercariae are larvae of renicolid species and postulated that species of Renicola may have two types of cercariae, either rhodometopous or plagiorchid larvae. Stunkard (1964) described plagiorchid cercariae from Thais lapiltus and their development in the renal tubules of gulls, Larus argentatus , to adults that were identified as a new species, Renicola thaidus . He noted that these cercariae are similar to others from Littorina spp., viz., Cercaria parvicaudata , Stunkard and Shaw, 1931, and Cercaria roscovita , Stunkard, 1932. Werding (1969) reported that metacercariae of C. roscovita developed in the kidneys of L. argentatus to adults identified as Renicola roscovita (Stunkard, 1932). He suggested the identity of C. roscovita and C. parvicaudata . Robson and Williams (1970) identified cercariae from Littorina littorea as Cercaria A ; the larvae agreed morphologically with C. roscovita , but attempts to infect juvenile L. argentatus , ducklings, chicks and various laboratory mammals failed. Repeated attempts by the writer to infect L. argentatus and other birds with metacercariae of C. parvicauda have given only negative results. The systematic position of the rhodometopodous cercariae is enigmatic, but the inclusion of such morphologically diverse types as the plagiorchid and rhodometopodous cercariae in a single genus controverts ideas of generic unity.