Water flow inside the shell of Mytilus galloprovincialis was measured by phase-contrast magnetic resonance imaging (MRI). In seawater without algal cells at 23°C, water approached the mussel from the posterior-ventral side, and entered through the inhalant aperture at a velocity of 40–20 mm s −1 . The flow rate in the lower mantle cavity decreased to 10–20 mm s −1 , the water flowed in the anterior–dorsal direction and approached the demibranches at a velocity of 5–10 mm s −1 . After passing through the lamellae to the upper mantle cavity, the water stretched the interlamellar cavity, turned to the posterior–dorsal direction and accumulated in the epibranchial cavity. The water flows came together at the ventral side of the posterior adductor muscle. The velocity increased more to than 50 mm s −1 in the exhalant siphon, and exhaled out in the posterior–dorsal direction. The anterior–posterior direction of the flow was imaged every 1.92 s by the inflow effect of T 1 -weighted MRI. The flow seemed to be constant, and no cyclic motion of the mantles or the gills was detected. Spontaneous closure of the shells caused a quick drop of the flow in the mantle cavity. In the opening process of the shells, water flow in the interlamellar cavities increased before the opening, followed by an increase of flows in the exhalant siphon and inhalant aperture with minimum delay, reaching a plateau within 1 min of the shells opening. This provides direct evidence that the lateral cilia drive water in the mussel M. galloprovincialis .