Post, T. J. and T. Uzzell (Department of Vertebrate Biology, Academy of Natural Sciences, Philadelphia, Pennsylvania 19103) 1980. The Relationships of Rana sylvatica and the Monophyly of the Rana boylii Group. Syst. Zool., 30:170-180.-Reciprocal immunological distances were measured using antisera to the serum albumins of Rana sylvatica and representatives of five groups of Holarctic Rana (R. pipiens, R. catesbeiana, R. temporaria, R. lessonae, and R. muscosa or R. aurora or both). Scaled reciprocal values for three data sets (with R. muscosa, with R. aurora, or with both) were used to generate three sets of Fitch and Margoliash phenograms. The nine lowest percent standard deviations ranged from 6.18 to 8.44, 5.20 to 6.95, and 7.51 to 9.12, respectively. The interspecific distances in both the input and network matrices suggest that R. sylvatica is immunologically most like the eastern North American Rana. Probable cladograms place R. sylvatica either with the eastern North American Rana or in an independent lineage between the brown-frogs and the water frogs. Oneway comparisons with antigens representing 12 additional species were made using the first four antisera listed. The R. sylvatica antiserum distinguished two subgroups of eastern North American Rana while the R. temporaria antiserum did not, and the R. temporaria antiserum distinguished two subgroups of both Nearctic (Rana boylii group) and Palearctic brown frogs while the R. sylvatica antiserum did not. Our data support the monophyly of the Rana boylii group, the affinity of the Palearctic brown frogs to the R. boylii group, and a distant association between R. sylvatica and the eastern North American Rana. [Microcomplement fixation; Holarctic Rana; paraphyly.]