Structure and behaviour of the sperm terminal filament.  Light- and electron-microscopic observations of Ciona and Lytechinus spermatozoa show a thin terminal filament at the distal end. The terminal filament is 5-6 microns long and contains the two central microtubules and a variable number of A-tubule extensions of the peripheral doublet microtubules. The transition from the 9 + 2 region to the terminal filament is tapered more gradually in Lytechinus than in Ciona. Photographs of the movement of beating spermatozoa do not show any obvious discontinuity in curvature at the transition region. Bends are propagated smoothly off the end of the flagellum with no decrease in curvature. However, spermatozoa in which the terminal filament has been removed show a clear ''end effect''. This end effect involves a rapid unbending of bends that have reached the distal end of the flagellum. Computer simulations of flagellar models lacking a terminal filament show a similar end effect. Addition of a terminal filament to the end of the computer model can eliminate the end effect. Realistic bending behaviour of the model is obtained by using a terminal filament with a tapered elastic bending resistance in the basal portion of the terminal filament and a value of 0.03 x 10(9) pN nm2 in the remainder of the terminal filament. This leads to estimates of 0.01 x 10(9) pN nm2 for the elastic bending resistance of an individual microtubule, and 0.2 x 10(9) pN nm2 for the elastic bending resistance of the 9 + 2 region of the flagellum. An improvement in propulsive effectiveness by addition of a terminal filament remains to be demonstrated.