Four trials were conducted to evaluate the nutritional characteristics of a methanol-based yeast single-cell protein (YSCP) for chickens. In experiment 1, the relative availability of phosphorus from YSCP was determined by using weight gain, percentage femur ash, and femur ash as criteria. The relative availability of phosphorus from YSCP was 69.8% as compared with 100% for tricalcium phosphate;Experiments 2 and 3 were conducted to evaluate YSCP as a source of amino acids for laying hens. In both experiments, YSCP was substituted for 0, 33, 66 or 100% of the protein of soybean meal (SBM). Pooled data from the first seven 28-day periods of experiment 2 showed that replacing SBM with YSCP did not affect egg production but reduced egg weight, egg yield, feed consumption and body weight gain. At the end of the seventh period, methionine supplementation was increased to .13, .22, and .28% in diets containing 6.55, 12.90 and 19.15% YSCP, respectively. This resulted in an improvement in egg yield for the hens fed 6.55 or 12.90% YSCP diets, but the hens fed 19.15% YSCP diet showed no improvement in egg yield. In experiment 2, mortality increased with increasing increments of dietary YSCP. This relationship was postulated to be the result of a deficiency of selenium (Se). The results of experiment 3 showed that neither level of YSCP nor supplemental Se (0.1 ppm) had a significant effect on laying performance. No relationships were observed between the levels of YSCP or supplemented Se and rate of mortality;Experiment 4 was conducted to evaluate YSCP as a source of amino acids for chickens over two generations (F0 and F1). YSCP was substituted for 0, 33, 66 or 100% of the dietary SBM protein. Replacing 100% of the SBM protein with the YSCP during starter phase significantly reduced average daily gain and feed intake. Growth during the grower and the breeder phases was not affected by inclusion of YSCP in the diets. Inclusion of YSCP in the diets during the breeder phase did not significantly affect productivity of hens or hatchability of eggs. However, substituting YSCP for 66 or 100% of the dietary SBM protein reduced fertility slightly. No physical abnormality in embryos or in live chicks was observed that could be attributed to dietary YSCP. Mortality was low throughout the experiment and was not related to dietary YSCP.