Phylogenetic analysis of sequences of the cell division gene ftsZ retrieved from the Atlantic coast revealed an interesting subgroup NAC11-7, which was targeted by a specifically designed and optimized Taqman assay in diel samples collected in situ and in parallel on-deck incubations. Rapid changes of ftsZ gene copies and the patchy distribution of other phylotypes at different time points suggested that different NAC11-7 populations were sampled. Strong correlations between ftsZ expression and gene abundance (r-squared=0.62), and between ftsZ expression and water temperature (r-squared=0.73) for in situ samples suggested non-synchronous growth of NAC11-7 group. Contrastingly, a sharp 9:00 AM peak of ftsZ expression in the on deck incubation experiment suggested synchronous growth. We propose a possible mixed model in which a certain fraction of the population is synchronously dividing, while a background of asynchronously dividing NAC11-7 cells also exist, some of which are expressing ftsZ at any given time.