  We report that an instability boundary of a single-mode state in Kerr ring
microresonators with ultrahigh quality factors breaks the parameter space span
by the pump laser power and frequency into a sequence of narrow in frequency
and broad in power resonance domains - Arnold tongues. Arnold resonances are
located between the Lugiato-Lefever (lower) and universal (higher) thresholds.
Pump power estimates corresponding to the universal threshold are elaborated in
details. RF-spectra generated within the tongues reveal a transition between
the repetition-rate locked and unlocked regimes of the side-band generation.
