Galactosemia is an autosomal recessive disorder of human galactose metabolism caused by deficiency of the enzyme galactose-1-phosphate uridyl transferase (GALT). The molecular basis of this disorder is at present not well understood. We report here two missense mutations which result in low or undetectable enzymatic activity. First, we identified at nucleotide 591 a transition which substitutes glutamine 188 by arginine. The mutated glutamine is not only highly conserved in evolution (conserved also in Escherichia coli and Saccharomyces cerevisiae), but is also two amino acid residues downstream from the active site histidine-proline-histidine triad and results in about 10% of normal enzymatic activity. The arginine 188 mutation is the most common galactosemia mutation characterized to date. It accounts for one-fourth of the galactosemia alleles studied. Second, we report the substitution of arginine 333 by tryptophan, caused by a transition at nucleotide 1025. The area surrounding this missense mutation is the most highly conserved domain in the homologous enzymes from E. coli, yeast, and humans, and this mutation results in undetectable enzymatic activity, suggesting that this is a severe mutation. This second mutation appears to be rare, since it was found only in the patient we sequenced. Our data provide further evidence for the heterogeneity of galactosemia at the molecular level, heterogeneity which might be related to the variable clinical outcome observed in this disorder.