An analysis of unpublished phytoplankton data collected in 1919—1920 suggests that certain chain—forming diatoms, under field conditions, undergo synchronized cell division at night and suffer extensive daytime grazing mortality. If this interpretation of the data is correct, the average total daily grazing represents at least 40%, and possibly as much or more than 50% of the standing crop. Such rates would imply that turnover of phytoplankton is extremely rapid in the nearshore area studied.