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Continuous-wave operation of a distributed-feedback quantum cascade laser emitting at around 4.6 mm grown by solid source molecular beam epitaxy combined with metal organic vapour phase epitaxy is presented. These devices display an ultra-low threshold current density of 0.85 kA/cm 2 at 30 8C. The low threshold characteristics resulted in continuous-wave operating temperature up to 100 8C for thermoelectrically cooled devices and room-temperature continuouswave output power in excess of 8.5 mW for uncooled devices.Introduction: The distributed-feedback (DFB) quantum cascade laser (QCL) is a very efficient and versatile single-mode light source for infrared spectroscopy-based detection of molecular traces and gases in the atmosphere [1,2]. Rapid progress in improving the performance of DFB QCLs has been made in recent years. The high performance of DFB QCLs that combine room-temperature continuous-wave (CW) operation with high output power and high sidemode suppression ratio (SMSR) have been explored by a growing number of groups [3 -5]. In spite of this, there are some severe fabrication drawbacks. One of them is the high threshold current density of DFB QCLs operated in room-temperature CW mode, which has a typical value larger than 1 kA/cm 2 regardless of the active region design and growing techniques. Unfortunately, high threshold current density will result in the overheating of the device and then bring its failure. As a result, for better room-temperature CW operation of DFB QCLs, some additional means seem essential to enhance the heat dissipation, such as using semi-insulating InP regrowth to form the buried heterostructure, using AlN or diamond as the submount materials. The complicated process goes against the trend of low-cost lasers for mass production. Even so, room-temperature CW DFB QCLs must be achieved only with devices thermoelectrically (TE) cooled to operate, which require bulky power supplies and lead to the increase of power consumption. Although uncooled operation is more demanding for the DFB QCL chip than TE-cooled operation at room temperature, the lack of reliable high quality QCL wafers is embarrassing. How to prepare a high quality material in these QCLs and whether a low threshold device could be achieved are still open questions and are the topic of the present investigation.In the work reported in this Letter, an extremely good quality QCL wafer was grown by solid source molecular beam epitaxy (MBE) combined with metal organic vapour phase epitaxy (MOVPE). A simple device processing of DFB QCLs at l 4.6 mm is presented. These devices display an ultra-low threshold current density of 0.85 kA/cm 2 at 30 8C. Single-mode emission with an SMSR of 30 dB is achieved up to a temperature of 100 8C in CW mode for TE-cooled devices. In addition, the first uncooled DFB QCLs operated at room-temperature CW mode are also demonstrated.