This protocol consists of several methods for the preservation of seawater samples to later perform various techniques in the lab. It is meant as a sampling treatment in the field and is written as comprehensive; however, depending on needs and scientific questions, any or all of the methods may be used.Flow Cytometry (FCM): Samples are used for counting bacteria and for sorting different groups of microbes (eg, sorting synechococcus and prochlorococcus and non-fluorescent bacteria). Glutaraldehyde fixes microbes and renders them unculturable whereas microbes preserved in DMSO and Betaine can usually be cultured after thawing and rinsing. Betaine was developed for single cell sorting and may be a better preservative than DMSO.SYBR staining: Samples are used for counting bacteria (unfiltered or pre-filtered seawater) and viruses (unfiltered, pre-filtered or 0.2um filtered seawater) microscopically after staining with SYBR-Gold (or SYBR-Green). Transmission Electron Microscopy (TEM): Samples are used for determining frequency of visibly infected cells (FVIC) and determining types of microbes or viruses present, and can be quantitative.Quantitative PCR (qPCR): Samples are used for quantitative PCR of microbes. They can also be used for quantitative PCR of viruses.Viral qPCR: Samples are used for quantitative PCR of viruses.Culturing Viruses: Samples are used for cultivating viruses by solid or liquid plaque assay on susceptible hosts. Samples can also be used for viral tagging (using flow cytometry) and most probable number (MPN) assays.Fluorescence in situ hybridization (FISH): Samples are used with fluorescent probes to determine what populations of bacteria or other microbes are present. The filters may also be used to simply count the number of microbes in a sample. For detailed FISH procedure see here.