Populations of European freshwater pearl mussels, Margartitfera margaritifera, have declined dramatically throughout their range. Better knowledge of postlarval feeding is required to determine factors influencing juvenile survival and growth, both in captivity and in natural streams. Development of the filtering organ represents a critical stage in juveniles. The filtering organ (ctenidium) in postlarvae (shell length, L = 1–3 mm) was studied by quantitative histology. Ctenidial development depended on mussel size, not age and was not accelerated by exposing the postlarvae to a 100-d artificial winter. Three distinct ctenidial stages were identified: the I-, V- and W-stages, which describe the ctenidial shapes, starting at L = 0.8, 1.1 and 4.5 mm, respectively. Functional redundancy of pedal and filter feeding probably reduces winter mortality at I-stage and early V-stage. Filter feeding becomes efficient at c.L = 2.2 mm, when the filtration area reaches 2.1 mm2, inducing an ontogenetic shift from pedal feeding to ctenidial filter feeding. For every 123 µm increase in L, one additional filament develops posteriorly from a budding zone. Filament thickness (c. 28 µm) and the distance between them (c. 29 µm) do not change with mussel size, indicating that juveniles and adults capture the same food particles. The change from pedal feeding to filter feeding represents an ontogenetic feeding shift and hence represents a critical stage in survival of juvenile M. margaritifera.