In order to determine small amount of13C abundance in Daphnia magna (contains 10-50 μg of carbon), calibration curve have been studied in detail for the semimicroanalysis of infrared absorption method. The standard glycine samples used for this calibration were one fifth scale compared with the ordinary one (0-200 μg of natural carbon at 20 μg intervals) . The13C abundances of known samples (1.07, 5.40, 9.79 atom% ; about 30 μg of C) determined by the new calibration curves fitted well to the theoretical values. The differences from theoretical values were less than 4%.