Insect tracheal taenidia contain a keratin‐like protein *

. The presence in insect tracheal taenidia of a protein having an immunochemical determinant common to vertebrate keratins is suggested. Taenidia react positively with a specific anti keratin serum both at the fluorescent and electron microscope level. In tracheal preparations, immunoblotting shows specificity for three polypeptides having molecular weights ranging from 62 to 43 kd, two of them corresponding to prekeratins. In the region of taenidia reacting with the antisera, 7–8 nm thick filaments are present. The occurrence of a keratin‐like protein in insects, and the role played in the tracheal intima are discussed.