A wide variety of inducible expression systems have been designed for Gram-negative bacteria, but adapting these systems to phylogenetically distinct species, such as mycobacteria, has proved notoriously difficult. Mycobacteria belong to a class of high G+C Gram-positive bacteria known as actinomycetes. Although comparatively few genetic tools are available for these organisms, those that do exist are more likely to be adaptable for use in mycobacteria. A compelling example of this rationale is the recent description of a tetracycline-responsive element from corynebacteria that functions in mycobacteria. Here we describe the use of two additional mycobacterial expression systems that are derived from endogenous regulons of Streptomyces and Rhodococcus spp. Each of the currently available systems has specific advantages and limitations, and the conditions that recommend the use of each will be discussed.