Cell ultrastructure can be used as a diagnostic indicator for the mechanism(s) of plant cell elongation. Young cotton fibers, between 3 and 7 days post anthesis (DPA), were examined with electron microscopy, using both conventional glutaraldehyde fixation and rapid freeze-freeze substitution (RF-FS) fixation. Quantitative analysis of organelle distribution the tip and subapical regions of developing cotton fibers shows that while Golgi bodies and mitochondria are evenly distributed throughout the cytoplasm, secretary vesicles appear to bind preferentially to the cell apex. Cell wall microfibrils are transversely oriented in the inner most layer of the wall and axially oriented it the outer most region of the wall. The distribution of Golgi bodies, mitochondria and wall mircrofibris is consistent with an intercalary mechanism of fiber elongation. The distribution of secretory vesicles indicates that tip synthesis may occur. It appears therefor that during the early stages of elongation fibers grow via a combination of tip and intercalary synthesis.