Chaotic behaviors in loss-modulated erbium-doped fiber-ring lasers are reported. In a single-frequency loss-modulated erbium-doped fiber-ring laser, the coexistence of period-doubling bifurcation and intermittency routes to chaos is observed by changing the modulation frequency or the modulation voltage. The results are in agreement with those in an erbium-doped fiber-ring laser subjected to pump modulation. Dual-frequency loss-modulated erbium-doped fiber-ring lasers are proposed. When the value of the frequency ratio is approximately equal to the gold mean and the silver mean, respectively, the transition to chaos via quasiperiodicity is found when the modulation voltage is increased. The modulation voltages for the transition are different for the varied values of the frequency ratio.