Fatigue crack growth behavior was obtained for the SAEFDE Committee's round-robin A356-T6 cast aluminum alloy program with crack growth rates between 10−11 and 10−6 m/cycle for R-ratios equal to 0.1 and 0.5. Three different mold temperatures resulted in secondary dendrite arm spacings (DAS) that varied from approximately 80 to 90 µm, resulting in only coarse microstructure. Threshold levels, ΔKth, and the Paris exponent, m, were approximately twice the values usually found for wrought aluminum alloys. The influence of R-ratio was quite pronounced and crack closure, as measured with a crack mouth COD gage, did not eliminate all threshold and near-threshold R-ratio differences. Roughness-induced crack closure appeared to be more important than plasticity-induced closure.