Marine algal toxins of the okadaic acid group can occur as fatty acid esters in blue mussels, and are commonly determined indirectly by transformation to their parent toxins by alkaline hydrolysis. Some data are available regarding the identity of the fatty acid esters, mainly of palmitic acid (16:0) derivatives of okadaic acid (OA), dinophysistoxin-1 (DTX1) and dinophysistoxin-2 (DTX2). Other fatty acid derivatives have been described, but with limited mass spectral data. In this paper, the mass spectral characterization of the [M-H](-) and [M+Na](+) ions of 16 fatty acid derivatives of each of OA, DTX1 and DTX2 is presented. The characteristic fragmentation of [M+Na](+) ions of OA analogues provided a useful tool for identifying these, and has not been described previously. In addition, a set of negative ion multiple reaction monitoring (MRM) methods was developed for direct determination of 16 fatty acid esters of OA, 16 fatty acid esters of DTX1 and 16 fatty acid esters of DTX2 in shellfish extracts. The MRM methods were employed to study the profiles of fatty acid esters of OA analogues in blue mussels and to compare these with fatty acid ester profiles reported for other groups of marine algal toxins.