Alcaligenes eutrophus JMP 134 was continuously (carbon-source-limited) grown on phenol to determine the maximum growth rates (mu(max)) as a function of the phenol assimilation pathways expressed. During growth on phenol as the sole source of carbon and energy, an almost exclusive expression of the ortho cleavage pathway (catechol 1,2-dioxygenase) was observed at initially low growth rates. This allowed a mu(max) of 0.28 h(-1). The induction of the meta cleavage pathway (catechol 2,3-dioxygenase), which appeared at around 0.25 h(-1), resulted in a further increase in the growth rate to 0.40 h(-1) after the enzyme activities of this pathway had been correspondingly expressed. Hence, two maximum growth rates, one for the ortho and one for the meta cleavage pathway, exist for the growth of A. eutrophus JMP 134 on phenol. Growth on phenol was stimulated by formate, which served as an auxiliary energy source in this strain. The simultaneous utilization of phenol and formate at a molar ratio of 1:5.2 resulted in an increase of the yield coefficient from about 0.75 g dry mass/g phenol to 1.25 g/g. Furthermore, formate exerted a pronounced effect on the growth rate. At a molar ratio of phenol to formate of 1:4.2, the growth rate was increased to 0.42 h(-1), despite the exclusive induction of the ortho cleavage pathway. The meta cleavage pathway was expressed during growth on this substrate mixture at about 0.4 h(-1). However, this did not enable a significant increase of the growth rate beyond 0.4 h(-1). This is attributed to an exhaustion of the capacity for formate oxidation at this rate. The results are discussed with respect to energy production capabilities when phenol is assimilated as an energy-deficient heterotrophic substrate.