We have isolated and studied cytochromic-deficient mutants of the amylolytic yeast Schwanniomyces castellii in order to study the possible contribution of cytochromes to alternative pathways. Three mutants were found, lacking cytochrome b, a + a3, or b and a + a3. All strains presented two alternative pathways, which were induced in the wild strain when cytochromic respiration was suppressed by growth in the presence of inhibitors, or without copper. If cytochromic respiration was absent, the Yxs yields in aerobiosis were higher than in anaerobiosis. This shows that the alternative pathways play a part in energy conservation. Cytochrome a + a3 did not appear to be directly involved in the alternative pathways.