Milk protein polymorphisms in yaks, cattle and their hybrids were examined in the Solu district, Nepal. Simultaneous phenotyping of αs1-casei (αs1-Cn), β-casein (β-Cn), κ-casein (κ-Cn) and β-lactoglobulin (β-Lg) by isoelectric focusing revealed restriction in gene flow between yaks and cattle as was found in the Khumbu district in the previous study. Traditional breeding by Sherpa may have discriminated among types of animals properly and their mating program seems to have been managed under a strict rule on the hybrid production.