Fusobacterium nucleatum, grown in a chemically defined medium at micro(rel) = 0.5, produced greater cell yields and undetectable levels of intracellular polyglucose (IP) when fructose was substituted for glucose. The utilisation and metabolism of fructose by growing cells was studied and the effect of the energy-yielding amino acids, glutamate, serine, histidine and lysine on cell yield, IP synthesis and acidic end-products was investigated. When F. nucleatum was grown on elevated amino acid levels, IP was synthesised from fructose and amino acids were metabolised to lactate, acetate, butyrate and formate. Under these conditions, IP synthesis was associated with the cells being replete with amino acid-derived energy; an observation supported by the absence of IP when the levels of (energy yielding) amino acids were reduced. Compared with fructose, glucose was less efficiently removed from the growth medium and produced less biomass and markedly lower levels of IP during energy-limited growth.