AbstractDetergent‐isolated flagellar apparatuses of the flagellate green algaDunaliella bioculataretain remnants of the nucleus (the karyoskeleton) which are linked to the basal bodies by the centrin‐containing nucleus basal body connectors (NBBC). Such complexes were subjected to different calcium concentrations (1 × 10−9M − 5 × 10−4M Ca2+) and the distance between the basal bodies and the karyoskeleton was measured by light microscopy. The threshold concentration of Ca2+for NBBC contraction was determined to be around 5 × 10−8M Ca2+. At>10−7M Ca2+NBBC were maximally contracted and the distance between the basal bodies and the karyoskeleton was only about 50% of the initial distance. Using a polyclonal antibody generated against centrin (Salisbury et al., 1984), the NBBC were visualized by indirect immunofluorescence in both the extended and contracted state. Our results demonstrate thatin vitrocontraction of centrin‐containing filaments in green algae is initiated at about the same free Ca2+concentration in three different centrin‐containing basal apparatus components (i.e. the distal connecting fibre, the flagellar transitional