Sucrose-sucrose I-fructosyltransferase (l-SST) was purified lOO-fold from tubers of Hefiunthus tuberosus L. The purified enzyme was essentially devoid of invertase activity and could be separated by isoelectric focusing into five isoforms which all were composed of two subunits (59 and 26 kDa). When incubated individually with sucrose, l-FFT was inactive while l-SST formed isokestose (trimer) and, upon prolonged incubation, some nystose (tetramer). When a combination of the two enzymes was incubated with sucrose, a series of oligofructosides with a degree of polymerization of up to 20 was formed. Amino acid sequences of tryptlc peptide fragments from both l-SST and l-FFT indicate that these enzymes are highly homologous with plant invertases.