The circadian clock is an endogenous timer that oscillates with a period of approximately 24 hours and is reset upon environmental time cues such as the daily light/ dark or temperature cycles. Chlamydomonas reinhardtii is an ideal model organism for research on the circadian clock, because it shows several well-characterized behaviors that exhibit a circadian rhythm. Its circadian rhythm of phototaxis (swimming toward light) has been automated. Former action spectrum studies using the circadian phototaxis rhythm as an indicator surprisingly found that pulses of blue light were not effective in resetting the circadian clock of dark-adapted cells. This may have been because of the particular strain used in the study (the cell wall-deficient strain CW15). It may also have been due to the additional phase shift caused by the act of placing the cultures into the monitoring machine at particular times during their circadian cycle. This additional phase shift was most likely the result of the white background light present when monitoring the rhythm of phototaxis. The phototaxis monitoring process was improved by using narrow-wavelength LEDs specific for phototaxis as test lights and by omitting the background light between test light cycles. This study demonstrates that the modifications prevent any phase shifts due to the cultures being placed into the monitoring machine. Using a further improved experimental set-up and the wild-type strain CC124, this study unambiguously shows that blue light of 440nm is effective in resetting the circadian clock in Chlamydomonas reinhardtii. Because of this difference in blue light response to the earlier study, the action spectrum of the entire visible light range was also evaluated. Effective wavelengths for resetting the circadian clock in wild-type C. reinhardtii were found to occur at 400nm, 440nm, 540nm, and 640-660nm, corresponding to near UV-A, blue, green, and red light, respectively. With the exception of 440nm, these findings are congruent with previous action spectrum studies for the cell wall-deficient strain CW15.