Amino acid requirements for the growth of Aedes albopictus, clone C6/36, cells and for the production of dengue (DEN) and Chikungunya (CHIK) viruses were examined by growing the cells or the viruses in media which were deprived of one of the 20 amino acids. Cell growth was markedly inhibited when cystine was omitted from the medium, and to a lesser extent by arginine deprivation. On the other hand, omission of alanine, asparagine, aspartic acid, and glutamic acid at the same time did not affect cell growth.Marked accumulation of alanine was observed in the medium when the cells were grown for 8 days in complete medium, with concomitant depletion of aspartic acid and glutamic acid. The production of CHIK virus was inhibited markedly by omission of cystine from the medium after virus infection, while the production of DEN viruses was more affected by glycine deprivation, although cystine deprivation also inhibited virus production to a lesser extent. On the other hand, production of CHIK and DEN viruses was not affected when alanine, asparagine, aspartic acid, and glutamic acid were omitted from the medium at the same time.Dengue (DEN) virus (a flavivirus, family Togaviridae) and Chikungunya (CHIK) virus (an alphavirus, family Togaviridae) are mosquito-borne arboviruses (3, 4), and infections with these viruses are important public health problems in Southeast Asia (10, 11). Studies on the growth characteristics and modes of existence of these viruses in vector mosquitoes are required to understand the basic principles of their ecology and the epidemiology of the diseases caused by the viruses. We have been studying the growth of these viruses in cultured mosquito cells, which can be considered a simplified system comparable to the vector mosquitoes. Recently, a clone, C6/36, (12) was isolated from a line of Singh's Aedes albopictus cells (23), and was shown to be more sensitive to DEN and CHIK viruses than the original uncloned cells.