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Phytoplankton samples were collected from the West Pacific Sector of the Southern Ocean to measure the growth rate from November 30 to December 1, 1995. Prorocentrum scuttellum was selected for growth rate measurement using the method of cell cycle analysis. During the 24 hr sampling cycle, cells of P. scuttellum changed from 2,500 to 5,000 cells/L. The highest abundance was observed at 8:40 AM, December 1, and lowest at 11:40 PM, November 30. Cellular division seemed to occur sometime between 11:40 PM, November 30 and 2:40 AM, December 1. After cell division, DNA fluorescence shifted slowly towards the right, representing the S phase, and the majority of the cells were in S+G2 phases at 8:40 AM, December 1. Between the next six hours, a sharp drop in DNA fluorescence occurred, representing mitosis, and the majority of the cells returned to the Gl phase by 2:40 PM, December 1. We can not determine the duration time of the terminal event from this result However, the growth rate of P. scuttellum was calculated as 0.43d^-1 with the help of curve fitting methods. This unexpected result seems to have resulted due to background noise, unsynchronous cell division, unequal sampling, water column unstability, and migrating behavior of P. scuttellum.
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