If freshly dried tomato pomace could be fed to poultry, its naturally occurring alpha-tocopherol would retard lipid oxidation during further processing, long-term frozen storage, and heating of poultry meat; however, the high fiber content in this agricultural by-product adversely affects its use. Experiments were conducted to investigate the chemical composition and in vitro true digestibility of amended (without and with 487 micromol of manganese/g) tomato pomace substrate after treatment with the white-rot fungus Pleurotus ostreatus. In treated pomace without manganese, protein content was improved by 3.1%, cellulose and hemicellulose decreased over time, but lignin degradation was not detected. In addition, treated pomace without manganese showed a significant reduction of in vitro true digestibility. Manganese in pomace inhibited fungal growth and did not enhance lignin degradation. Under the conditions of the experiment, P. ostreatus improved the nutritional composition of tomato pomace; however, it did not reduce lignin. It is possible that manganese amendment at the level used affected gaseous conditions (O(2) consumption and CO(2) evolution rates), important factors that must be considered when attempting to enhance accelerated lignin degradation by P. ostreatus.