The temperature and relative humidity preferences and tolerances of two Texas species of cave-adapted millipedes, Cambala speobia (Chamberlin) and Speodesmus bicornourus Causey, were studied. Both species showed gross preferences when tested in gradient chambers for temperatures and relative humidities approximating those of their cave environments. But C. speobia, the less adapted species morphologically, was the more selective of the two species for such conditions. S. bicornourus was far less tolerant of elevated temperatures and reduced relative humidities than was C. speobia. Discussed is a possible reason why a terrestrial troglobite like S. bicornourus would combine intolerance with a lessened ability to perceive those factors to which it is intolerant. Discussed also are the possible causes of the present distribution of Cambala and Speodesmus in the caves of central Texas.