The oceans are a small source of non-methane hydrocarbons (NMHC). Previous work has established a photochemical source in the water column for many alkenes, and a phytoplanktonic source for isoprene. The focus of this work was to gain further insight on marine microbiological cycling of NMHC. A variety of phytoplankton species were examined for the ability to produce isoprene in laboratory cultures. All were found to have constant isoprene production rates per cell during exponential growth, with decreasing rates as the populations reached stationary phase. Production rates ranged from approximately 1x10-21 to 2x10-18 moles (cell)-1 (day)-1 for the different species. A positive allometric correlation between isoprene production rate and cell volume was found; highest production rates per cell were found for the largest cell tested, Emiliania huxleyi, and lowest rates for Prochlorococcus, the smallest. Isoprene production by Prochlorococcus was found to be a function of light intensity and temperature, with patterns similar to the relationships between growth rate of this species and these environmental parameters. Grazing (by Cafeteria roenbergensis) and cyanophage infection of Prochlorococcus both caused cell mortality, and thus the total amount of isoprene produced declined. While isoprene production per cell remained constant in the grazed cultures, there was a decrease in isoprene production per cell associated with the latent stage of cyanophage infection. We also regularly monitored 5 other NMHC, but detected no clear production or consumption of ethane, ethene, propene, 2-mepropene, or hexane from any of the phytoplankton or heterotrophic organisms tested. Total isoprene production in the water column was estimated for both oligotrophic and North Atlantic ocean regions using reported in situ measurements of cell abundance and our laboratory production rates. These estimates are on the order of 1 to 10 (x107 molecules isoprene (cm)-2 (sec)-1), which is similar to previously reported estimates of ocean-atmosphere isoprene flux based on measured surface water and air concentrations. Inclusion of the light and temperature dependencies of cellular isoprene production may influence these estimates. As expected, isoprene production rates from phytoplankton are small compared to those from higher plants, but may be sufficient to affect the chemistry of the remote atmospheric boundary layer.