  We report the elastic properties of the Kondo semimetal CeBi by resonant
ultrasound spectroscopy measurements at zero magnetic field. Clear elastic
softening is found in bulk modulus $C_B$ below $\sim 60$ K. Such a softening in
$C_B$, in addition to the anomalous temperature dependent Poisson's ratio, is
hardly attributable to multipolar response for stable localized $4f$ orbital,
but can be well described by a two-band model arising from the hybridization
between conduction- and $4f$- electrons. These results probably are
consequences of the valence fluctuations in this Kondo semimetal as originally
suggested by a Fermi-surface expansion observed in a previous angle-resolved
photoemission spectroscopy study [P. Li \textit{et al.}, Phys. Rev. B
$\mathbf{100}$, 155110 (2019)].
