Interrelationships and possible hybridization between the shiners (CYPRINIDAE) Notropis lutrensis and N. venustus (subgenus Cyprinella) were studied in Bayou Pierre, Mississippi. Three approaches were used to evaluate these interrelationships. The first approach consisted of discriminant analysis of meristic and morphometric characters to identify parental and possible hybrid individuals in preserved specimens. This discriminant function separated the two reference N. lutrensis and N. venustus groups with 100% accuracy. An analysis of quarterly fish collections at eight localities showed a variable frequency of apparent hybrid individuals with an average of 13.7% The second methodology consisted of discriminant analysis for the separation of the above three groups on the basis of physico-chemical parameters. This approach was used to estimate niche overlap and separation between parental and hybrid clusters. The results showed the apparent hybrid group to be intermediate between parental groups, and Notropis lutrensis and N. venustus to have a niche overlap of 22.3%. Separation of the groups was primarily based on turbidity, apparent color, and the depth relative to width of the collecting site The final approach consisted of electrophoretic analysis to investigate genetic structure in Notropis collections and to explore possible genetic introgression between Notropis lutrensis and N. venustus. Comparison of proteins from allopatric and sympatric populations suggested likely gene introgression into N. venustus from N. lutrensis at several presumed genetic loci (CK, S-Mdh-2, Got-2) in the Lower Mississippi drainage. The occurrence of hybridization and introgression is also supported by the higher levels of genetic variability in Lower Mississippi N. venustus compared to levels in allopatric populations Hybridization between the two species is suggested as the origin of Gibbs' (1957) 'Red-Lower Mississippi' race of N. venustus. There is also evidence of gene introgession into N. lutrensis from N. venustus, although results are less certain due to higher background levels of genetic variability in N. lutrensis Although the three analytical approaches suggest a variable level of hybridization and gene introgression in Bayou Pierre, evidence indicates that Notropis lutrensis and N. venustus are generally distinct from each other in areas of sympatry, and are associated with specific allelic combinations. This distinctness suggests selection for the maintenance of certain gene complexes, and the specific validity of N. lutrensis and N. venustus. Hybridization in Bayou Pierre appears to be related to habitat alteration, increased turbidity, and to generalized breeding behavior in the two species Notropis camurus, a common member of the subgenus Cyprinella in Bayou Pierre, was also compared to its congeners. Electrophoretic evidence suggests that the species is quite distinct from both N. lutrensis and N. venustus, although it is closer genetically to N. venustus. A low frequency of hybridization occurs between N. camurus and N. venustus, although there is no apparent genetic introgression occurring