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A subset of olfactory receptors (ORs) is expressed in mammalian male germ cells. Recent studies on human and mouse sperm have suggested that calcium signaling via a testicular OR regulates sperm flagellar motility. However, it remains to be determined at what stages testicular ORs are expressed during spermatogenesis and whether each germ cell expresses one or multiple ORs. Here we examined the developmental expression profiles of several mouse testicular OR genes using an in situ hybridization technique at the cellular level. We found that OR transcripts in the spermatogenic cells are expressed in three developmental stages: late pachyten spermatocytes, early round spermatids, or late round spermatids. The OR mRNAs were condensed in a single dot-like structure within the nuclei of a subpopulation of spermatogenic cells. Double-fluorescent in situ hybridization revealed that some cells contained two dot-like signals derived from transcripts of two different ORs, suggesting that single spermatogenic cells could express more than one OR. One cell-multiple OR gene expression combined with variability in expression appears to result in heterogeneity in the repertoire of ORs expressed by individual spermatogenic cells. Although the functional consequence of heterogeneous OR expression awaits development of a methodology for characterizing OR proteins, our observations give insights into OR gene expression as well as OR function(s) in spermatogenic cells.