Hybridization is not uncommon and can occur between closely related species within virtually any fish family (Hubbs 1955; Schwartz 1981; Jenkins and Burkhead 1994). Among nonindigenous fishes, there is evidence of both artificial and natural hybrids occurring in open waters. Our records on introduced hybrids are obviously incomplete. Even though there are many unpublished reports documenting introductions of hybrids, the identity of stocked hybrids is not always certain. The possibility of back-crosses further complicates the issue. Nevertheless, hybridization and introgression between introduced and native fishes, some of which are endangered or threatened, is an issue of increasing concern. Hybridization between introduced and native species is known to result in loss of genetic purity and reduced reproductive output in native fish populations. The consequences are especially serious when it involves rare species or subspecies (e.g., Echelle and Connor 1989; Behnke 1992). Introgression (incorporation of genes from one population into another) is also a concern because it may lead to the breakdown of locally adapted gene complexes and loss of genetic diversity (e.g., Hindar et al. 1991). As such, we felt it important to provide some of the more common or unusual cases involving hybrid introductions, both artificial and natural.