A liquid chromatography/mass spectrometry (LC/MS) method has been applied to the analysis of the osmolyte glycine betaine and its biosynthetic precursor choline from marine particulates containing natural assemblages of marine plankton. The limits of detection (5.3 and 4.6 pg injected on column, for glycine betaine and choline, respectively), far exceeded that obtained by high performance liquid chromatography with ultraviolet (UV) detection (47 ng injected on column) used previously to quantify glycine betaine in microalgal cultures and marine coastal particulates. The increased sensitivity of detection obtained using an electrospray ionization (ESI) LC/MS approach vastly reduces the sampling volume required to detect glycine betaine and choline in seawater particulates, enabling studies in less productive waters. In addition, the LC/MS mobile phase composition is compatible with commonly used glycine betaine extraction solvents, which simplifies and shortens the extraction process. The LC/MS method also avoids reliance on the nonspecific UV absorbance at 195 nm for osmolyte detection, characteristic of commonly used high performance liquid chromatography methods, resulting in increased confidence in peak identification.