Allele-frequency differences at 11 polymorphic protein loci provided discrete genetic markers with which to estimate the extent of hybridization among native and introduced centrarchid basses in central Texas streams. Native Guadalupe bassMicropterus treculiand northern largemouth bassM. salmoides salmoidesnow coexist with introduced Florida largemouth bassM. s. floridanusand smallmouth bassM. dolomieuiin these streams. Interspecific hybridization was detected in three of four populations from the Blanco and San Marcos rivers, hybrids making up at least 31.8% of the individuals sampled. Complex hybridization patterns were evident, because F1, F2, and backcross hybrids were detected. At least one individual exhibited genetic markers of largemouth, smallmouth, and Guadalupe bass. Extensive multispecies hybridization threatens the survival of the endemic Guadalupe bass.