  Transition probabilities of the ground-state bands in $\gamma$-soft nuclei
are studied for the first time using the triaxial projected shell model
approach. It is observed that the angular-momentum dependence of the transition
quadrupole moment $Q_t$ is related to the triaxial deformation of the nuclear
mean-field potential. The introduction of the $\gamma$-degree of freedom in the
shell model basis is shown to have a little influence on the {\it constant}
behavior of the low-spin $Q_t$ in a well-deformed nucleus. However, the {\it
increasing} collectivity with spin for the low-spin states in a $\gamma$-soft
nucleus can only be explained by considering the triaxial mean-field
deformation.
