. . "However, hybridization between Mexican spadefoot toads and plains spadefoot toads is well known (H.A. Brown, 1976a; Sattler, 1985); in some areas, such as the San Simon Valley, Arizona, high frequencies of F1 hybrids and backcross progeny occur, with hybrid adults representing up to 31% of the breeding congress at some pools (Simovich et al., 1991; Simovich, 1994)." . .