We demonstrate thermometry with a resolution of 80 nK= ffiffiffiffiffiffi Hz p using an isotropic crystalline whispering-gallery mode resonator based on a dichroic dual-mode technique. We simultaneously excite two modes that have a mode frequency ratio that is very close to two (AE0.3 ppm). The wavelength and temperature dependence of the refractive index means that the frequency difference between these modes is an ultrasensitive proxy of the resonator temperature. This approach to temperature sensing automatically suppresses sensitivity to thermal expansion and vibrationally induced changes of the resonator. We also demonstrate active suppression of temperature fluctuations in the resonator by controlling the intensity of the driving laser. The residual temperature fluctuations are shown to be below the limits set by fundamental thermodynamic fluctuations of the resonator material. DOI: 10.1103/PhysRevLett.112.160801 PACS numbers: 07.20.Dt, 42.60.Da, 42.62.Fi The high-resolution measurement of energy has long fascinated humans with its culmination seen in ultra-highsensitivity calorimeters [1,2] and bolometers [3]. These and related ideas have found a broad range of applications, including bolometric superconducting photon counters for quantum communication [4] and ultrasensitive radio astronomy [5,6]. The record for absolute thermometric sensitivity has been realized at cryogenic temperatures, achieving better than 100 pK= ffiffiffiffiffiffi Hz p [7]. In this Letter, we develop a new method to measure temperature based on excitation of two well-separated modes in a millimeter-scale whispering-gallery (WG) optical resonator. WG mode resonators have exceptionally high Q factors and can provide the potential of providing high-stability microwave and optical signals [8][9][10][11][12]. Recently, they have been applied to high-sensitivity label-free sensors for molecules and viruses [13,14] and for optical comb generation [15]. Nonetheless, an issue that afflicts all of these applications is the high temperature sensitivity of WG resonators [12,16], particularly when compared to conventional vacuum-spaced Fabry-Perot resonators [17][18][19][20][21]. In this Letter, we turn this problem to our advantage by using the WG resonator as an ultrasensitive thermometer.To suppress unwanted temperature fluctuations in WG resonators, several groups have demonstrated in situ thermometry by measuring the frequency difference between two orthogonally polarized modes. The best of these techniques have demonstrated a resolution of ∼100 nK= ffiffiffiffiffiffi Hz p [22], and subsequent temperature stabilization based on this sensing has resulted in improvement to the long-term frequency stability [23,24]. In contrast, we present a two-color approach to measure the resonator temperature with high resolution. In comparison to the birefringent dual-mode technique, our approach can be used in both anisotropic and isotropic resonators, which expands the range of material candidates. Isotropic materials have shown the highest Q facto...