The high-cycle fatigue behavior of several nickel and cobalt-base eutectic composites is described. Major test variables include test temperature, frequency, alloy composition, and microstructural modifications induced by postsolidification heat-treatments. Lamellar γ/γ′-δ type eutectic alloys are shown to have superior fatigue resistance to the fibrous alloys, with regard to both total life and crack propagation rates, provided that the microstructures are perfectly aligned. Cracking at 25°C in all alloys tends to be along crystallographic planes (Stage I cracking), although striations associated with Stage II propagation were occasionally observed in solution-treated Nitac. At 825°C there is no evidence of Stage I crack growth or Cotac or Nitac. A pronounced frequency effect occurs in Nitac at 825°C (0.7 T_m) in vacuum, suggesting a creep-fatigue interaction.