For the purpose of investigating the effect of nalidixic acid on the nucleic acid synthesis in chloroplast isolated from Chlorella ellipsoidea, cells were cultured in the media treated with nalidixic acid(20ppm) for 5 days. Aliquots cells were taken out at the inoculation and at intervals during the culture and growth rate of Chlorella cells measured. After extraction of nucleic acids in chloroplast isolated from these cells, their contents were analyzed by the base composition and the effect of nalidixic acid on the nucleic acid synthesis interpreted to compare with those of the control. 1. It was showed that the inhibitory concentration affected by nalidixic acid on the growth of Chlorella cells were 20ppm. 2. Because nalidixic acid had depressed the DNA replication in isolated chloroplast as well as whole cell system, these contents were markedly decreased in comparison with those of the control. 3. In the isolated chloroplast as well as in the whole cell system, nalidixic acid was decreased contents of base in the RNA by preventing RNA transcription.