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ABSTRACT DYNAMIC simulation of methane fermentation of animal waste has advanced with the development of a more accurate and comprehensive mathematical model that considers and describes two basic biological pathways of methane formation. Recent work in the field of microbology has shown that there are four major microbial cultures involved in methanogenesis, with the final pathways to methane formation involving acetic acid metabolism and the reduction of carbon dioxide with hydrogen. Digestion failure, often encountered using animal waste because of the high organic and nitrogen content, can only be predicted when the dynamic relationship of these four microbial groups are considered in a symbiotic manner. Work reported here describes a dynamic computer model that predicts digestor operating conditions for the four major animal types. Validation of the computer predicted operating conditions was performed using the steady-state data of 16 pilot and full-scale methane fermentation plants (four each of swine, dairy, beef and poultry). Upon validation of the model using these data, projected minimum deten-tion times at specific loading rates for temperatures of 35 C and 60 C were determined. Relationships developed in this model provide some insight into the basic response, under dynamic conditions, of biological processes.