Within each stereocilium of chick hair cells is a hexagonally packed bundle of actin filaments. Diffraction patterns of thin sections of these bundles reveal that the actin filaments are aligned such that the crossover points of adjacent filaments are in transverse register. Since each actin filament is composed of subunits that are organized in a helical pattern, yet all the actin filaments are in transverse register, crossbridges between filaments can form only at positions dictated by the geometry of the actin helix or at 125 A intervals. Thus the crossbridges appear in electron micrographs as regularly spaced bands (125 A) that are perpendicular to the axis of the stereocilium. From examination of stereocilia of organisms who have a temporary threshold shift due to exposure to loud noise, we know that the integrity of the actin filaments and their crossbridges is essential for hair cell function. However, particularly interesting is that when a stereocilium is bent or displaced, as might occur during stimulation by sound, the actin filaments are not compressed or stretched, but slide past one another so that the bridges become tilted relative to the long axis of the actin filament bundle. Thus, resistance to bending or displacement must be a property of the number of bridges present which in turn is a function of the number and lengths of actin filaments present. Since hair cells in different parts of the cochlea have stereocilia of different, yet predictable lengths and widths, this means that the force needed to displace the stereocilia of hair cells located at different regions of the cochlea will not be the same. This suggests that fine tuning of the hair cells must be a built-in property of the stereocilia. To try to understand how hair cells control the length and number of actin filaments per stereocilium and thus the length and width of the stereocilia, we examined cochlea in chick embryos of increasing maturity. Of interest is that very early in development (10-day embryos) the total hair cell number and position is specified. Thus it is possible to study the growth of stereocilia in cells whose final stereociliary length and width is already known. Stereocilia first elongate (from 8 to 11 days--first phase); they then stop elongating and increase in width (12-16 days--second phase), then elongate again (third phase) to the length appropriate to the position of the hair cell on the cochlea. During the first phase a few actin filaments are present, but initially poorly ordered.(ABSTRACT TRUNCATED AT 400 WORDS)