ABSTRACT The flexural rigidity of the large abfrontal cilia of Mytilus has been measured with a flexible glass micro-needle. The same cilium has similar values to the flexural rigidity irrespective of the phases of beat cycle (including the recovery phase) and of the direction of force applied. The values of 3–13 × 10−8 dyne.cm2 have been obtained for the flexural rigidity of compound cilia of various sizes; 2·3 × 10−10 dyne.cm2 for that of the component cilia. The Young’s modulus of the microtubule is estimated to be 5–9 × 1010 dyne/cm2 on the basis that the outer doublet microtubules are tightly connected with one another.