The cathepsins are a family of cysteine proteases that have been broadly implicated in proteolytic processes during cell growth, cell development, and normal adult cellular function. Cathepsin L is a major secretory product of rat and mouse Sertoli cells, the absence of which in furless mice is associated with atrophy of some seminiferous tubules. However, furless mice produce viable sperm, suggesting the possibility that other members of the cathepsin family of proteases may complement cathepsin L action in the testis. Our objective herein was to begin to test this hypothesis. To this end, we first utilized cDNA microarray technology to identify the members of the cathepsin gene family expressed by freshly isolated adult rat Sertoli cells. This approach, complemented by Northern blot analyses, showed that in addition to cathepsin L, cathepsin K is highly and specifically expressed in Sertoli cells. As is also true of cathepsin L, cathepsin K mRNA was found to be expressed by Sertoli cells at specific stages of the cycle of the seminiferous epithelium, with maximal expression at stages VI–VII. The use of immunocytochemical methods revealed that cathepsin K protein localizes to the cytoplasm of Sertoli cells at stages VI–VIII, to small punctuate lysosomes at stages I–VIII and XIII–XIV, and to early and late residual bodies at stages IX–XII. This localization was found to be similar to that of cathepsin L. The similarity in the expression and localization of cathepsin K and cathepsin L suggest that the two proteases may have similar functions. If true, this might explain the fertility of furless mice. Further, the results suggest that cathepsin K, in both its secreted and lysosomal forms, may play a role in the degradation of Sertoli cell residual bodies.