A study of the steady-state kinetics of fumarase over an extended concentration range, using novel methods of analysis, reveals an initial-rate equation of at least fourth degree for malate as substrate at pH 7.0, with no kinetically significant dead-end complex formation even up to concentrations of 100 mM. In the absence of demonstrable enzyme-aggregation phenomena, this is interpreted as indicating co-operative effects overlooked previously, although a mixture of isoenzymes, each individually of high degree and giving a complex curve, may be a contributing factor.