The oil-producing fungus Mortierella alpina 1S-4 is an industrial strain for arachidonic acid (AA) production. To determine its physiological properties and to clarify the biosynthetic pathways for PUFA, heterologous and homologous gene expression systems were established in this fungus. The first trial was performed with an enhanced green fluorescent protein gene to assess the transformation efficiency for heterologous gene expression. As a result, strong fluorescence was observed in the spores of the obtained transformant, suggesting that the foreign gene was inherited by the spores. The next trial was performed with a homologous PUFA elongase (GLELOp) gene, this enzyme having been reported to catalyze the elongation of GLA (18:3n-6) to dihomo-gamma-linolenic acid (20:3n-6), and to be the rate-limiting step of AA production. The FA composition of the transformant was different from that of the host strain: The GLA content was decreased whereas that of AA was increased. These data support the hypothesis that the GLELOp enzyme plays an important role in PUFA synthesis, and may indicate how to control PUFA biosynthesis.