Results from a 4-year tagging study of the migration behavior of Pacific salmon smolts indicate an effect of lunar gravitation on the time of saltwater entry at an artificial tidal barrier. We used passive integrated transponders to tag several populations of juvenile Chinook salmon Oncorhynchus tshawytscha, coho salmon O. kisutch, and sockeye salmon O. nerka, including hatchery and wild Chinook salmon. We discovered through exploratory data analyses that the temporal variation in the date of initiation of passage at the barrier by the majority of the Chinook salmon populations studied was most closely related to the date of lunar apogee, followed by the date of quarter moon. Both phenomena are associated with weaker net lunar gravitation effects on water bodies than the full or new moon, which showed no consistent effect. Using available data we could not discern any effect of other environmental variables, including water temperature, nocturnal illumination, salinity, and lake level. Lunar apogee occurred on average 1 d prior to the onset of substantial passage by Chinook salmon smolts. The phenomenon was consistent in all 4 years, even though the lunar apogee cycle advanced annually by 7 d. An intermediate relationship was noted for coho salmon smolts and the weakest relationship for sockeye salmon smolts