Pavlova lutheri, a marine microalga, is rich in the very long chain polyunsaturated fatty acids (VLCPUFAs) eicosapentaenoic (20:5n-3) and docosahexaenoic (22:6n-3) acids. Using an expressed sequence tag approach, we isolated a cDNA designated Pldes1, and encoding an amino acid sequence showing high similarity with polyunsaturated fatty acid front-end desaturases. Heterologous expression in yeast demonstrated that PlDES1 desaturated 22:5n-3 and 22:4n-6 into 22:6n-3 and 22:5n-6 respectively, and was equally active on both substrates. Thus, PlDES1 is a novel VLCPUFA Delta4-desaturase. Pldes1 expression is four-fold higher during the mid-exponential phase of growth compared to late exponential and stationary phases.