Cells properly respond to extracellular stimuli and circumstantial environment. The unicellular eukaryotic protozoan Tetrahymena is a potentially useful animal cell model system for studying the molecular mechanism of adaptation to environment. Tetrahymena is exposed to fluctuations in temperature, pH, amounts of nutrients and concentration of dissolved gases in natural habitat. For example, the cells adapt to cold environment by increase in unsaturated fatty acids in membrane phospholipids to maintain proper membrane fluidity. To accomplish this modification, the activity of fatty acid desaturase is increased upon a down-shift in temperature. We have cloned delta9 fatty acid desaturase which is involved in this process and shown evidence that its mRNA level increased in response to cold environment. Moreover, in order to examine other genes responsive to clod stress, we have adopted mRNA differential display technique to temperature shift-down of T. thermophila. We have cloned two kinase genes, NIMA (never-in-mitosis in Aspergillus nidulans)-related protein kinase (TpNrk) and MAP kinase-related kinase (MRK). Interestingly, these genes were also shown to be expressed by the osmotic stress.