The effects of salinity on the phototaxic and geotaxic response of C. gigas larvae have been studied in an air conditioned laboratory. The results indicated that a rise in salinity might promote positive photoresponse and change positive geotaxis into negative response. The threshold of salinity increase for strengthening significantly the photoresponse of veliger larvae was 2.9×10^-12, whereas in the veliconcha stage (8-day), this threshold increased to more than 5×10^-12. The time threshold for increasing obviously the photoresponse of veliger and veliconcha larvae were 4 h and 6 h, respectively, when they increased from 25×10^-12 to 30×10^-12. It is therefore suggested that the activeness of larvae staying in the estuary likely depends on the environmental variation in the estuary as well as on the larval stage. The haloclinic effects on the vertical distribution of C. gigas larvae have been determined. Larvae of C. gigas might reduce their upward movement greatly as soon as a halocline was constructed even if the magnitude were as small as 2.5×10^-12. Further increase in the halocline had nearly the same effect as that of a low halocline. This result suggests that the salinity stratification in nature is likely to inhibit the vertical migration of larvae in this species.