Pseudochoricystis ellipsoidea is a recently isolated unicellular green alga, which is classified within the family Trebouxiophyceae. This alga has a unique ability to synthesize and accumulate intracellularly a significant amount of aliphatic hydrocarbons. To elucidate molecular mechanisms of the hydrocarbon production in this organism, the development of genetic methods including DNA transformation methods are important. Towards the goal, we constructed several plasmids in which neomycin phosphotransferase II-encoding G418-resistant gene (nptII) is flanked by a P. ellipsoidea-derived promoter and terminator. These plasmids were introduced into P. ellipsoidea cells through particle-gun bombardment, and transformants were screened among G418-resistant cells by PCR amplification of plasmid-borne genes. Southern blot analysis demonstrated that the exogenous DNA was integrated into the genome of the transformants. Furthermore, the expression of nptII was confirmed at the transcript and protein levels by RT-PCR and immunoblot analyses, respectively. These results clearly indicated that a genetic transformation system was successfully established for P. ellipsoidea.