Eggs, larvae, and early juveniles of Catostomus commersoni, Hypentelium nigricans, Moxostoma macrolepidotum, Carpiodes cyprinus, and Erimyzon oblongus are described. Taxonomic keys are provided. Comparisons of developmental traits reveal two groups of larvae based on size at a given developmental stage. The larger species, the first three above, are very similar to one another. The other two are not difficult to distinguish. The large size of the posterior gas bladder chamber in C. commersoni is unique. Hypentelium nigricans has distinctive pigmentation in the dorsal and caudal fins. Moxostoma macrolepidotum is typically deeper bodied than the others. Catostomus commersoni is the most filiform of the large species. Erimyzon oblongus has a median pigment-free space on the dorsum which is not present in the other species. Combinations of morphometric parameters are used as key characteristics. Construction of these characters takes into account the high degree of allometric growth observed in these larvae. Discriminant function analysis is employed to assist in differentiation of these larvae. The resulting procedure yields satisfactory results but the process is tedious and is not practical for large samples. Laboratory growth illustrates the greater variability in size at a given age over wild specimens. The value of various types of taxonomic characters used in larval fish studies is discussed. Possible evolutionary relationships suggested by larval characteristics differ somewhat from the phyletic structure devised from adult catostomid morphology.
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