The effect of hydrocarbon yeast grown on n-paraffin, as dietary protein source on growth, production and reproducibility of hens was studied for three generations.<br>Two hundred female chicks as the first generation were grouped into 8 lots of 25 chicks each and four experimental diets containing 0, 10, 15 and 20% of hydrocarbon yeast in place of all of fish meal and part of soybean meal in the basal diet were fed to the duplicated lots. Fifty female chicks each hatched from the eggs layed by the hens on each of 4 experimental diets were reared as the second generation, feeding the same experimental diet as that fed to their mother under the same experimental plan. Fifty chicks each as the third generation were reared similarly as their mother and grand mother.<br>Analyzing the data of 3 generations statistically, the following results are obtained:<br>(1) Body weight at 20 weeks of age and feed intake for 20 weeks decreased in inverse proportion to the dietary level of yeast, though little difference was observed in feed conversion among the 4 experimental diets.<br>(2) Little difference was observed on the sexual maturity, i.e. age at the first egg.<br>(3) Little difference was observed on feed intake and feed conversion during laying stage from 20 to 52 weeks of age. Egg production tended to increase in proportion to dietary yeast level, though the trend was not confirmed statistically. Due to the higher egg production on the yeast diet, little difference was observed on egg output per hen per day among the 4 experimental lots, although average egg weight on the yeast diets was significantly smaller than that on the control diet.<br>(4) Dietary hydrocarbon yeast had little effect on the mortality of the hens during growing and laying stages. All of the hens were healthy. No abnormality due to the yeast feeding was observed by post-mortem examination of the hens died during the experimental period.<br>(5) Little difference was observed on fertility. Hatchability of fertile eggs laid by the hens of the first generation fed the diets containing yeast was much poorer than that on the control diet. It was confirmed that the poor hatchability on yeast diet was simply due to the lack of vitamin B₁₂ in the yeast diets, as reported previously⁶⁾. With the supplementation of 50μg of vitamin B₁₂ per kg of diet, little difference on hatchability was observed among the eggs laied by the hens of the second and third generations. No deformed chick was found among the chicks hatched.