An investigation of the serological relationships of five species of clupeoid fishes: alewife, Alosa pseudoharengus; blueback herring, Alosa aestivalis; American shad, Alosa sapidissima; Atlantic herring, Clupea harengus harengus; Atlantic menhaden, Brevoortia tyrannus, was made by four methods: (a) photronreflectometer, (b) agar diffusion, (c) erythrocyte agglutination with absorbed antisera, and (d) paper electrophoresis. Agar diffusion provided qualitative differentiation of the species tested, while the photronreflectometer provided a quantitative measure of relative serological distances between species; results from the two methods were in good agreement. On the basis of results obtained by the photronreflectometer and agar diffusion methods, the following species relationships are indicated: alewives and bluebacks are very closely related; shad lie quite close to alewives and bluebacks, but farther from them than alewives and bluebacks are from one another; menhaden and herring are further removed from the alewife-blueback-shad complex, with menhaden closer to it than herring; herring are comparatively remote from the other four species. Generalized electrophorelic patterns were found for each clupeoid species. However, because of intra-species variability, paper electrophoresis does not seem to be as useful a procedure for determining relationships of fishes as immunological methods. Species-specific patterns should be proposed only after large numbers of individuals representing both sexes have been sampled under different physiological conditions. Absorptions of rabbit antisera with pooled erythrocytes of each of the five clupeoid species indicated that alewife and blueback were antigenically very similar; menhaden and shad were antigenically somewhat similar, although not as close as alewife and blueback.