Cells of CW-15 mutant of Chlamydomonas reinhardtii without a cell wall were transformed by electroporation. The hpt gene of hygromycin phosphotransferase was used as a selective marker. Optimal conditions of transformation were observed in the middle of the logarithmic growth phase at the density of suspension 10(6) cells/ml, electric field intensity 1 kV/cm, and pulse duration 2 ms. Under these conditions up to 10(3) hygromycin-resistant clones of trasformants per 10(6) recipient cells were obtained that was 100 times higher than at the usage of wild-type cells. Exogenic DNA integrated into the genome of the nucleus C reinhardtii was constantly inherited for more than 350 generations. The use of mutants without a cell wall and certain selective systems enable the efficiency of transformant yield to be doubled problems on unstable expression of geterologous genes to be investigated, and ways of obtaining super producers of foreign proteins using the alga C. reinhardtii investigated. (C) 2003 Elsevier Ltd. All rights reserved.