The cell surface of the epimastigote form of Trypanosoma cruzi is covered by glycoconjugates rich in galactose. The parasite cannot take up galactose through its hexose transporter, suggesting that the epimerisation of UDP-glucose to UDP-galactose may be the parasite's only route to this sugar. The T. cruzi UDP-glucose 4'-epimerase is encoded by the TcGALE gene. We were unable to make a CL-Brener strain T. cruzi epimastigote TcGALE-/- null mutant, suggesting that the gene is essential. Two TcGALE+/- single-allele knockout clones displayed aberrant morphology and haploid deficiency with respect to galactose metabolism. The morphological phenotypes included shortened flagella, increased incidence of spheromastigotes, agglutination and a novel walnut-like appearance. The reduced supply of UDP-galactose was manifest in the two clones as a six- or nine-fold reduction in the expression of galactopyranose-containing cell surface mucins and negligible or two-fold reduction in the expression of galactofuranose-containing glycoinositolphospholipids. The major loss of mucins as opposed to glycoinositolphospholipids may indicate that the latter are more important for basic parasite survival in culture. The apparent haploid deficiency suggests that epimerase levels are close to limiting, at least in the epimastigote form, and might be exploited as a potential drug target.