  Proton decay from the 3$\hbar\omega$ isoscalar giant dipole resonance (ISGDR)
in $^{58}$Ni has been measured using the ($\alpha,\alpha'p$) reaction at a
bombarding energy of 386 MeV to investigate its decay properties. We have
extracted the ISGDR strength under the coincidence condition between
inelastically scattered $\alpha$ particles at forward angles and decay protons
emitted at backward angles. Branching ratios for proton decay to low-lying
states of $^{57}$Co have been determined, and the results compared to
predictions of recent continuum-RPA calculations. The final-state spectra of
protons decaying to the low-lying states in $^{57}$Co were analyzed for a more
detailed understanding of the structure of the ISGDR. It is found that there
are differences in the structure of the ISGDR as a function of excitation
energy.
