Laboratory incubation experiments and shipboard measurements on the Southern Atlantic Ocean have provided the first evidence for marine production of monoterpenes. Nine marine phytoplankton monocultures were investigated using a GC-MS equipped with an enantiomerically-selective column and found to emit at rates, expressed as nmol C(10)H(16) (monoterpene) g [chlorophyll a](-1) day(-1), from 0.3 nmol g [chlorophyll a](-1) day(-1) for Skeletonema costatum and Emiliania huxleyi to 225.9 nmol g [chlorophyll a](-1) day(-1) for Dunaliella tertiolecta. Nine monoterpenes were identified in the sample and not in the control, namely: (-)-/(+)-pinene, myrcene, (+)-camphene, (-)-sabinene, (+)-3-carene, (-)-pinene, (-)-limonene and p-ocimene. In addition, shipboard measurements of monoterpenes in air were made in January - March 2007, over the South Atlantic Ocean. Monoterpenes were detected in marine air sufficiently far from land as to exclude influence from terrestrial sources. Maximum levels of 100-200 pptv total monoterpenes were encountered when the ship crossed an active phytoplankton bloom, whereas in low chlorophyll regions monoterpenes were mostly below detection limit.