The cif1 mutation of Saccharomyces cerevisia (Navon et al., Biochemistry18, 4487–4499, 1979) causes inability to grow on glucose and absence of catabolite inactivation. We have cloned the CIF1 gene by complementation of funcion and licated it in a 2·75 kb SphI-BstEII fragment situated at ca. 18 kb centomere distal of LYS2 and ca. 80 kb centromere proximal of TYRI on chromosome II. Southern analysis demostrated that CIF1 is present in a single copy in the yeast genome. Northern analysis revealed that the corresponding mRNA of 1·8 kb is more abundant in cells grown on galactose than in those grown on glucose. A protein of ca. 54 kDa was predicted from the open reading frame in the sequenced fragment. In strains carrying the cif1 mutation the intracellular concentration of ATP decreased immediately after addition of glucose while the intracellular concentration of cAMP did not increse. cAMP concentration increases in response to galactose or 2,4-dinitrophenol. Disruption of BCY1 or overexpression of CDC25 in a cif1/, background did not restore growth on glucose, suggesting that the absence of cAMP signal is not primary cause of lack of growth on glucose. Complementation tests showed that cif1 is not allelic to fdp1 although the two genes seem to be functionally related.