Nineteen pathogen free male rats were exposed to a ceramic fiber dust cloud with an average of 709 f/mL for 5 days, 6 hours per day, by nose only, to study the deposition of fibers in the different lung lobes. The count median diameter and length of the airborne fibers were 0.53 micron and 3.7 microns respectively. Five days after the last day of exposure, the rats were sacrificed; lungs were excised, and lobes were separated, weighed and digested. The digested lungs were filtered on membrane filters. These filters as well as samples of the airborne fiber cloud were evaluated by optical phase contrast microscopy at 1000 X total magnification. Fiber deposition was calculated from the concentration of airborne fibers, the minute volume, the duration of exposure and the number of fibers in the digested lung tissue for various fiber length and diameter categories. Fiber deposition in all the lobes varied over a narrow range, 5.43% for the right diaphragmatic lobe to 8.38% for the right apical lobe. The fiber burden for all lobes was found to be weight dependent. For the right apical lobe, the anatomical characteristics also influenced fiber deposition. Fiber size distributions in the various lobes were not significantly different.