Translation elongation factor 1α (EF-1α) catalyzes the GTP-dependent binding of aminoacyl-tRNA to ribosomes. We previously reported that Tetrahymena EF-1α induced the formation of bundles of rabbit skeletal muscle filamentous actin (F-actin) as well as Tetrahymena F-actin [Kurasawa et al. (1996) Zool. Sci. (Tokyo) 13, 371-375], and that Ca²⁺/calmodulin (CaM) regulated the F-actin-bundling activity of EF-1α [Kurasawa et al. (1996) J. Biochem. 119, 791-798]. In the present study, we investigated the binding between Tetrahymena EF-1α and CaM using a Tetrahymena EF-1α affinity column, and the localization of EF-1α and CaM by indirect immunofluorescence. Only CaM in the Tetrahymena cell extract bound to Tetrahymena EF-1α in a Ca²⁺-dependent manner. In interphase Tetrahymena cells, EF-1α and CaM are colocalized in the crescent structure of the oral apparatus and the apical ring, while in dividing cells, they are colocalized in the division furrow. This is the first report describing the coexistence of EF-1α and CaM in the division furrow, suggesting that EF-1α and CaM are involved in the organization of contractile ring microfilaments during cytokinesis.