We present a mark-release-recapture technique that can be used with a hand-held, portable Innov-X X-ray fluorescence spectrophotometer practical for taking to the field. Third instar A. gambiae larvae were reared in water treated with RbCl through pupation at concentrations of 0 to 1000ppm Rb. Larvae survived to adults at concentrations as high as 1000ppm Rb but did suffer pupal and adult mortality as the concentration of RbCl was increased. Similar comparitive studies were performed with A. stephensi. The effect of RbCl on adult longevity was determined and survival curves produced. The presence of Rb in adults was evaluated using the portable XRF analyzer, and we were able reliably to detect Rb above background in females after 10 days post-emergence and males after 4 days post-emergence at concentrations of minimal pupal or adult mortality. Rb marking does not, however, appear to be permanent. Removing the pupae from the Rb solution prior to emergence decreased detection of Rb in adults which indicates that Rb can be lost through diffusion. The low cost of labeling with RbCl and the field portability of the spectrometer provide a useful means for labeling mosquitoes via breeding sites or in the laboratory for numerous tracking experiments where marking has been problematic.