Zero-field muon spin relaxation studies are reported for crystalline YbxY1-xBiPt for x = 1.0 and 0.5. YbBiPt exhibits an extremely large Cp/T ratio y ≅ 8J/(K2 mol Yb) below T ≅ 0.2 K and shows evidence for some type of magnetic order below 0.4 K from χAC. The μSR data are consistent with a highly frustrated spin system segmented into two types of magnetic domains, one of which freezes into a disordered state below Tf ≅ 0.4-0.5 K at a temperature relatively independent of the Yb concentration. The frozen spin moment of ∼0.1μB is highly reduced from the high temperature free-ion value, and is independent of the Yb concentration. © 1993.