A small mismatch between natural frequencies of unidirectionally coupled chaotic oscillators can induce coherence resonance in the slave oscillator for a certain coupling strength. This surprising phenomenon resembles "stabilization of chaos by chaos," i.e., the chaotic driving applied to the chaotic system makes its dynamics more regular when the natural frequency of the slave oscillator is a little different than the natural frequency of the master oscillator. The coherence is characterized with the dominant component in the power spectrum of the slave oscillator, normalized standard deviations of both the peak amplitude and the interpeak interval, and Lyapunov exponents. The enhanced coherence is associated with increasing negative both the third and the fourth Lyapunov exponents, while the first and second exponents are always positive and zero, respectively. It is common knowledge that a nonlinear system in the presence of noise can exhibit resonance phenomena such as stochastic [1,2] or coherence [3] resonance. While the former is seen as an optimal response to external periodic modulation with respect to the noise intensity, the latter manifests itself as increasing regularity in one of the system internal time scales without additional modulation. Coherence resonance was detected first in excitable dynamical systems [3][4][5][6][7] and then in bistable systems [8][9][10]. Examples are typically found in biology in the form of neuron spiking dynamics [11].Analogous phenomena were observed in deterministic chaotic systems without noise, since an external chaotic forcing acts in a similar way as noise. Deterministic stochastic resonance was found in bistable chaotic systems [12][13][14] and deterministic coherence resonance in chaotic systems with delayed feedback [15][16][17] including diffusively coupled Rössler oscillators [18]. In laser applications, an increase in the injection diode current leads to an optimal regularity of chaotic laser diode power dropouts [15,16].It is not surprising that synchronization can affect chaotic dynamics [19]. Bragard et al. [20] observed chaos suppression in chaotic oscillators with bidirectional asymmetric coupling. They found that adequate asymmetry and coupling between two identical chaotic oscillators may force their dynamics towards regular periodic oscillations. Since this phenomenon occurs for the value of the coupling strength well below the value for complete synchronization, it was interpreted as a generalized synchronization state.In this Rapid Communication, we report on a significantly different case of deterministic coherence resonance. We consider two unidirectionally coupled nonidentical chaotic oscillators, masterẋ 1 = F(x 1 ,ω 1 ) and slaveẋ 2 = F(x 2 ,ω 2 ) + σ (x 1 − x 2 ), where x 1,2 are state variables of the master and slave systems, F is a vector function, and σ is a coupling strength. The oscillators are only distinguished by their natural * apisarch@cio.mx frequencies ω 1 and ω 2 . Due to nonlinearity, the dominant frequency ω 0 in the chaoti...