The literature provides several examples of biotransformations of microalgae. Microalgae have been used to improve the yield of already known substances or to produce novel substances [1]. Microalgae have been less considered in research on biotransformation and bioconversion of terpenes, with most interest in this area placed on conversion of monoterpene compounds. Earlier studies have already demonstrated the potential of microalgae for modification of steroids. In our own previous research, the biotransformation of onopordopicrin by Aspergillus niger was investigated; four novel products were obtained and their structures identified on the basis of chemical and spectroscopic data [2]. In other studies, biotransformation of citral, geraniol, menthol, myrcene, and nerol by various fungi, including A. niger and Penicillium spp., produced such compounds as 1,8-cineole, 2,6-dimethyloctane, -pinene, -terpineol, cis-p-menthan-7-ol, dihydrolinalool, -terpinene linalool, limonene, p-cymene, p-menth-1-ene, sabinene, and trans-p-menthan-1-ol [3–7]. In the present study, microbial transformation of the pure terpene alcohol menthol (1) was carried out using C. vulgaris. Biotransformation and bioconversion of menthol over varying periods of 72, 92, and 120 h produced primarily isomenthol. The product of the biotransformation was extracted with diethyl ether and analyzed with the use of Fourier transform infrared spectroscopy (FT-IR), gas chromatography (GC), and gas chromatography-mass spectroscopy (GC-MS). Biotransformation of menthol by C. vulgaris produced cis-p-menth-1-en-3-ol (2), dihydroterpineol, and isomenthol in high percentages. Similar products were obtained in previous research. The main bioconversion products of (–)-menthol by sporulated surface culture of Mucor ramannianus were trans-p-menthan-8-ol, trans-menth-2-en-1-ol, p-menthane-3, 8-diol isomenthol, sabinane and 1,8-cineole. Products obtained from menthol by surface grown Penicillium spp. included menthene (5.8%), terpineol (10.6%), -pinene (18.0%), sabinene (3.9%), 1,8-cineole (6.4%), and limonene (3.2%) [8]. Recent experimental work suggests that microbial transformation of monoterpenes with Penicillium spp. and A. niger involves oxidation, resulting in a more stable product. Using C. vulgaris for bioconversion makes possible to selectively obtain products selectively in high proportion. Using periods of 72, 92, and 120 h, we identified one or two major components for each time period, representing 69.8%, 92.3%, and 95.2% respectively. Menthol converted with C. vulgaris for 72 h produced dihydroterpineol (3, 48.8%) and isomenthol (4, 20.2%). When continued to 92 h, the main product was 4 (92.3%). At 120 h, the main compounds produced were cis-p-menth-1-en-3-ol (2, 46.0%) and 3 (49.2%) (Scheme 1). Scheme 1 shows the pathways involved in bioconversion of menthol to dihydroterpineol. Microalgae have been less considered in biotransformation and bioconversion of monoterpene compounds. In a previous study of biotransformation of menthol by sporulated surface cultures of A. niger, the main bioconversion product obtained was cis-p-menthan-7-ol. In a similar study using Penicillium spp. the primary products were limonene, p-cymene, and -terpinene [9]. Success has documented maximizing product yields by solubilizing or emulsifying immiscible substrates. In such a case, care should be paid to choice and concentration of the solvent as at lower concentrations many miscible solvents are cytotoxic [9]. Biotransformation of steroid estrogens by C. vulgaris has been investigated [5, 10]. Biotransformation of volatile monoterpenoids by fungi has also been investigated [3]. At each of these points volatile and monoterpene components mostly exit the first column. The best results were obtained at 92 h. For the other time periods, the percentage of the components was small and hence could not be identified. All were checked against the Eight Peak Index of Mass Spectra [11].