Gill cilia from the freshwater mussel Elliptio complanatus were examined after negative staining with phosphotungstic acid. The doublet and central pair microtubules typically collapse on the grid supporting film so that the protofilaments making up the wall of each tubule spread into a visible array. Thirteen protofilaments can be seen to form both the central pair microtubules and subfibre A of each doublet. Subfibre B of the doublet consists of 10 protofilaments.

As viewed in both normal and thiourea-treated cross-sectioned cilia, subfibre A appears as a complete microtubule with a circular profile and 26 nm diameter. Subfibre B is an incomplete, elliptically profiled tubule that shares, as a common wall or partition, 4 or 5 protofilaments of the A subfibre. The organization of the 3.5-nm thick protofilaments within the microtubules and their relationship to the various doublet appendages is discussed.

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