We report the first long baseline interferometric ob- servations obtained in the thermal infrared with a single-mode fiber coupler. Stellar fringes have been found onBootis (Arc- turus) andHerculis, with a 21 m baseline of the Infrared and Optical Telescope Array (IOTA), located at the Smithsonian's Fred Lawrence Whipple Observatory (Mt Hopkins, Arizona). Beams from the telescopes are coherently combined through an X shaped fluoride glass single-mode fiber coupler. Although this device was not originally optimized for operation in the L band (effective wavelength of 3.75µm), we find good interfero- metric properties in that wavelength domain, with a rather high and steady instrumental transfer function, and very low disper- sive effects. Observations are still limited by the poor sensitiv- ity of the detectors and not by the thermal background, which was minimized. Visibilities derived from the interferograms are consistent with the published limb darkened diameters ofHer- culis. It is, to our knowledge, the first time interferometric ob- servations of Arcturus andHer are reported in the L band.