Okadaic acid (OA) and its analogs, the dinophysistoxins, are potent inhibitors of protein phosphatases 1 and 2A. This action is well known to cause diarrhea and gastrointestinal symptons when the toxins reach the digestive tract by ingestion of mollusks. A less well-known effect of these group of toxins is their effect in the cytoskeleton. OA has been shown to stimulate cell motility, loss of stabilization of focal adhesions and a consequent loss of cytoskeletal organization due to an alteration in the tyrosine-phosphorylated state of the focal adhesion kinases and paxillin. OA causes cell rounding and loss of barrier properties through mechanisms that probably involve disruption of filamentous actin (F-actin) and/or hyperphosphorylation and activation of kinases that stimulate tight junction disassembly. Neither methyl okadaate (a weak phosphatase inhibitor) nor OA modify the total amount of F-actin, but both toxins cause similar changes in the F-actin cytoskeleton, with strong retraction and rounding, and in many cases cell detachment. OA and dinophysistoxin-1 (35S-methylokadaic acid) cause rapid changes in the structural organization of intermediate filaments, followed by a loss of microtubules, solubilization of intermediate filament proteins, and disruption of desmosomes. The detailed pathways that coordinate all these effects are not yet known.