Dormancy and migration to deep water allow oceanic copepods to escape unfavorable surface conditions including high mortality risk, low food availability, and advection out of favorable regions. Dormancy was examined in Calanus pacificus, a dominant California Current copepod. Hypotheses about onset of dormancy and transport and retention of dormant C. pacificus fifth copepodid (CV) stages were examined, including: (1) What is the seasonal pattern of variation in abundance of dormant copepods in deep, open water off southern California? (2) Are dormant copepods transported in the poleward California Undercurrent (CU)? (3) Is the abundance of dormant copepods different in basins and deep, open water? (4) When does onset of dormancy occur in surface water? and (5) What environmental conditions are associated with onset of dormancy? The vertical distribution and abundance of dormant C. pacificus were described over eleven months in 2000 – 2001 at the San Diego Trough (SDT) and at five basin and five open-water stations off southern California in January 1999 and October 1999 and 2000. Dormant C. pacificus abundance increased at the SDT from June until October, and decreased from October until March. The median depth and depth variability of dormant CVs increased while abundance was increasing, and both decreased while abundance was decreasing. Dormant CVs were observed mainly in the deeper half of the CU at the beginning and end of the dormant season and mainly in deeper water in the middle of the dormant season, indicating poleward transport for part of the dormant season. Vertical distributions at basin and other open-water stations in January and October suggest that the pattern of distribution observed at the SDT was widespread, except at the Santa Barbara Basin, a shallow basin. The abundance of dormant CVs was not different in basin and open-water stations. Variation in molting hormones was characterized through the CV molt cycle, and both molting-hormone level and jaw morphology were used as indicators of onset of dormancy in surface CVs. Although differences in mean molt-phase indices between the period of increasing deep CV abundance and other dates indicate onset of dormancy in surface water when deep CV abundance was increasing, variability in molt-phase indices among dates suggested a heterogeneous dormancy response in surface CVs. Onset of dormancy was observed at times of decreasing photoperiod, increasing temperature, and relatively high chlorophyll a.