Isochrysis galbana, a marine prymnesiophyte microalga, is rich in long chain polyunsaturated fatty acids such as docosahexaenoic acid (C22:6n-3, Delta(4,7,10,13,16,19)). We used a polymerase chain reaction-based strategy to isolate a cDNA, designated IgASE1, encoding a polyunsaturated fatty acid-elongating activity from I. galbana. The coding region of 263 amino acids predicts a protein of 30 kDa that shares only limited homology to animal and fungal proteins with elongating activity. Functional analysis of IgASE1, by expression in Saccharomyces cerevisiae, was used to determine its activity and substrate specificity. Transformed yeast cells specifically elongated the C18-Delta(9) polyunsaturated fatty acids, linoleic acid (C18:2n-6, Delta(9,12)) and alpha-linolenic acid (C18:3n-3, Delta(9,12,15)), to eicosadienoic acid (C20:2n-6, Delta(11,14)) and eicosatrienoic acid (C20:3n-3, Delta(11,14,17)), respectively. To our knowledge this is the first time such an elongating activity has been functionally characterised. The results also suggest that a major route for eicosapentaenoic acid (C20:5n-3, Delta(5,8,11,14,17)) and docosahexaenoic acid syntheses in I. galbana may involve a Delta(8) desaturation pathway.