A highly efficient system was developed for the expression of foreign genes in Chlorella ellipsoidea cells. The effect of five promoters on the expression efficiency of beta-glucuronidase (GUS) gene was evaluated by transient expression of the UidA gene. Among these promoters, Ubiquitin-omega was found to be the most efficient and was selected to drive the expression of foreign genes in Chlorella cells. A gene encoding the mature rabbit neutrophil peptide-1 (NP-1) was introduced into the cells. Integration of the gene for NP-1 into the Chlorella genome was confirmed by PCR and Southern blot analysis. In, vitro anti-microbial testes demonstrated the expression of biologically active NP-1 by the transgenic Chlorella cells.